I've got a two year old lab I dove/duck hunt with. One thing I haven't some a good job with is teaching him to heel..... Any advice or pointers on how to start and teach him?

There are more ways than one. Mine is to use a long lead with a pinch collar and start walking. A pinch collar is not a choke chain. Google the two if you don't know the difference. Just walk and give the dog plenty of lead. Over days I started decreesing the distance the dog could be from at heel position by grabbing the rope shorter and shorter. Soon you will be able to walk with the dog at the correct heel distance. When dog tries to get away from heel, give a clear command of heel and correct him with the pinch collar. Once that is established you can walk and stop, give a sit command. start and stop sit, start stop sit, start stop sit. Then you get to heel start walking stop and sit. Quuickly my dogs would just go to a sit at heel when I stopped walking. Some like that some don't. I like it.

In the early pup stages and teaching the pup the leash rules. I used a 2 foot lead and a pinch collar. I walked 2 yards in one direction then turned right, 20 yards again and turned right. You essentially make a square. Every day you walk that square and you take up 3 or 4 feet of lead rope. Eventually with not a lot of fight from the dog you will have them walking at heel. During this I say nothing, no commands. I just go. At times I had to wrap the lead rope around me to pull the dog along. He will give into the pich collar. Just keep going day after day without saying anything at all until he gets to heel. Then start teaching your heel commands.

I like mine to stay stuck to the last command. I do not use stay as a command. If I tell a dog to sit, I want them to stay seated until I give another command. I should not have to tell that dog to stay. Sit means sit, until I say otherwise. I find the less they have to think about the commmands you're giving, the better off both of you will be in the long run.

I also look at the command of "Sit" as a reset button. If a dog is doing a stupid, Sit is my go to command. That is the easiest for them to understand and follow. So when I give a sit command, my dogs know, they need to plant that bootie and find me with my eyes because he is about to tell me another command.

E-collar!

Bingo! Done! Even better if the e-collar has the nick and tone feature.

I'm guessing pup is a reliable retriever? What specific heel issues are you having?

Dog is an awesome retriever and doesn't run off. Even in a field he stays within 20 yards of me unless I tell him he's free.

My desire is when hunting public land or even just walking in the neighborhood I would like to have him glued to my side.

I don't like to use an e collar as a training device. I'm havin trouble getting him to understand walking next to me and how to reinforce that. It sounds like some lead rope work and constant heel correction is what's needed. But how to get that basic idea I him before I start with corrections?

Short quick, sideways pulls, and keep changing directions is what I do with mine when they are young. A pinch collar helps them get the idea a little faster.

Is he lagging or pulling?

General pulling unless he smells something.

I've always free walked him

OK. A great way to get the dog into a perfect heel position is to counteract what they are doing rather than yanking them around on the lead. For a pup that is pulling use pressure to enforce the sit command every time the dog forges ahead. I prefer to use the heeling stick for this rather than the collar because I find it more effective. If you put pressure on sit then the result is lagging which is the opposite reaction to the pulling. For a dog that lags, get in front of them and use pressure on "here". I prefer the e collar for this. It will take very few sessions for the dog to completely understand and then you can give the corrections while walking with the dog at heel. This method is very clear to the dog and prevents the handler from getting tennis elbow or their shoulder ripped out of the socket because you are letting your tools do the work instead of dragging the dog around.

Dogs like to pull. You should never allow them to pull. Give him a pinch with the pinch collar if you don't want to use the e-collar. Make sure you have the pinch collar on where it will pinch rather than choke. You want the short end of the collar to come over the top. Try it on your arm or leg to see the difference.

My dog will heel just fine. However, he will not walk backwards. If I walk backwards he circles and re-heels. Not really an issue except when re directing a mark.

Thoughts?

Put him on lead and find a wall or fence. Put the dog next to you in between yourself and the barrier and simultaneously give tugs backward on the lead and taps on his chest or front legs with the heeling stick. This will teach him to move straight back as he cannot turn his body.

This is the way I start training a dog to heel.
Put the dog on a 6 foot lead with a choke chain or pinch collar.Hold the leash by the looped handle allowing the slack to drag.
Start walking and just before the dog gets to the end of the leash jerk hard, say nothing, and walk in the opposite direction. The dog will follow you. Repeat this exercise until the dog no longer makes the leash tight. What this does is lets the dog know he has to pay attention to what direction you want to walk and not just go where it wants. You are also teaching the dog how to turn pressure off.
Now that he knows to quit moving forward when a correction is given we are ready to real him in and start the heel command.
Place the dog on your left or right side in a sit. Grab the lead and leave enough slack so that when the dog is walking next to you in the heel position there is no tension between your hand and the dogs neck. Say the word heel and start walking at a normal pace. Just before the dog uses up the allowed slack give a jerk, say heel, and keep walking. The only reason the leash should get tight is because you made it that way. In other words if the dog makes it tight you missed the opportunity to give a correction.
Once you have the dog heeling put an e collar on it along with your leash and choke chain. Try walking at a fast pace in heel and suddenly slowing down in order to trick the dog into messing up. If it does, give the known correction for heel paired with the new correction from the e collar. Gradually stop using the leash correction until all you have to use is collar pressure. Congratulations you now have a dog that heels without a leash on.
Good Luck.