I purchased a 20gauge for my son as his first shotgun. The barrel states it take 3inch shells however I cannot locate the proper ammo. Is it ok to assume that the gun can use 2.3/4 shells? It's for my 10 year old son and I don't want any accidents.. Thanks in advance

2 3/4 will me fine, for a 10 year old, better.

Yes. You can use a 2 3/4" in a 3". The marking indicates the longest approved shell for the gun and not the preferred length.

2-3/4 are plenty for me and I'm 25 and weigh 250 lol... unless your goose hunting then that's all you need

About the only time I use 3"s in my 20 ga is for ducks and I want to reach way out there.

Start your young hunter out shooting the 20 guage & using the light 7/8th oz dove loads, or better still the WW AA Skeet loads that should give better patterns. No need for the HV 1oz loads for doves, and only go to some larger shot sizes like 4's or 6's in the same 7/8ths oz loads for hunting ground game like rabbits, with shots taken at 30 yards max starting out on doves or rabbits using the Modified Choke, or the Improved Cylinder choke when bird hunting.

Hunting Waterfowl is whole 'nuther world and was where I first learned to hunt.... with different requirements, and using steel shot ammo exclusively that shoots totally differently than lead shot. However, Teal Season can be ideal for a just Starting Hunter, and an absolute blast when the birds are working your location and decoys. If you think dove hunting is fun then Teal are the same kind of shooting...on steriods.
Ron