I have heard that after 6mo of age button bucks are starting to produce semen. I dont know if this means they will breed for sure. Just what I heard. I know I have seen plenty getting the job done.
A June 12 born buck is possible for breeding, but a little more unlikely, than one born a few weeks earlier in May. Buck fawns are never a guarentee, even when they are big. Rubbed out buttons is the best chance though that he will be fertile.
We used a button buck that was born on May 18 to breed a mature doe two years ago and ended up with fawns in September. He may have been able to get the job done earlier if he was with a fawn instead of a mature doe.
If you are putting him with a big doe you might want to leave a step stool in the pen for him.