I'm with BB on this one. This buck is probably a 3 1/2 year old and COULD possibly be a 2 1/2 year old. One thing I have noticed about buck size in the USA is that the further south you go, the smaller the body (not rake). The farther north you go, the thicker the body and weight. Since you are in OK, which is fairly close to MO geographically, I would guess this buck to be a 3 1/2 year old. If that buck was here in north central MO, he'd probably be a 2 1/2 year old.
The thing that makes me think he's 3 1/2 is his belly- it droops down. A 2 1/2 year old buck isn't going to have that much droop. I definately know he's not a 4 1/2 year old b/c of his neck. 4 1/2 year old's will have a thicker neck than that.
he does have nice brows, which also makes me think he's 3 1/2 not 2 1/2.
That'll my official guess.... 3 1/2 this year and he'll definately grow bigger if that pic was taken on June 4. I would guess that he'll grow anywhere between 2-4 more inches before he's done for this year. Good luck to you and keep watching him!