
Thats really up to you when you want to have him done...Me personally I don't really see the need to fix a male (yes I have fixed 2 out of 3 but wont do it any more unless I have to) but if you are planning on it then I would wait till he is fully grown so he has the time to mature and fill out properly.
They do mark less and I do find they are less dominant when fixed, oya and no worries about them wanting to wonder for that bitch in heat and getting a bitch bred if fixed. lol :D
just because they are fixed does not mean they will not try humping, I have a bitch and dog fixed and they still hump when playing, I just tell them off and they stop real fast lol :D
Good luck :) The age limit is up to you, but please let him grow up and mature first :)