I agree, cut him now. Bulking up comes from working out. Temperment won't change much either, after he's gelded--MY 3 yr old gelding keeps testing me, and I have to remind him who's boss, and that "trust is a 2-way street, buddy!"
I once had a gelding who was easy to manage, transport--worming wasn't the best--great with shots, anybody could ride him, etc. I never met a horse that didn't back down to him. HE was the boss, everywhere he went, but he looked like a cuddly palomino teddy bear. Haven't had one myself THIS young, that wasn't already cut, but I'm shopping. I've talked to all my horsey friends, and they're all agreed--cut them by 6 months old.
I once had a gelding who was easy to manage, transport--worming wasn't the best--great with shots, anybody could ride him, etc. I never met a horse that didn't back down to him. HE was the boss, everywhere he went, but he looked like a cuddly palomino teddy bear. Haven't had one myself THIS young, that wasn't already cut, but I'm shopping. I've talked to all my horsey friends, and they're all agreed--cut them by 6 months old.