My GSP is 8, also 9 months. Health wise he is fine, and doing quite well with everything I ask of him. I don't have any measurements with me, but he is quite tall and slender. I have read that height wise they are done at around 10-12 months, but I have yet to find anything concrete about filling out weight wise. Anyone have any experience as to when I can expect him to start putting weight on and filling out a tad bit? Ill get some numbers when I get off work this evening.
AB

Mine is two and still skinny but starting to pad a litle

mine have always hit their "fighting weight" about 2-3 y/o and for the most part... they will only vary maybe 3 to 5 pounds from season to season throughout their lives unless you let them become couch potatoes. I have a 12 y/o now that only weighs about 3 pounds more than he did when he was 3 y/o and you can still count his back 3 or 4 ribs, He also still hunts all season long with me.

IMO, keeping them on the slender side is a good thing.