If he's already old enough to try to hump your arm, he's likely not going to start spraying. If that's your only problem, and even then it's isn't a usual thing for him, I wouldn't get him neutered - the procedure is very risky, as rabbits don't do well under anesthesia, and even then it's hard to find a vet with experience in rabbits surgeries. BTW, olafthebunny, rabbits do spray humans - they aim for the face too, the little jerks. I don't keep anybody who sprays me, that's one behavior I will not put up with.
1. He is starting to hump my arm (2 weeks ago he started). Does that mean he's sexually mature? Is this a good time to neuter him? - He is probably sexually mature. They reach sexual maturity very quickly.
2. He hasn't sprayed me or anyone does that mean he won't start? Will he still spray if neutered if he starts? - He probably won't start spraying. He probably won't if neutered too.
3. If he isn't neutered, does he have to have a "bunny" friend (stuff animal) to do his thing with? Is this required for his happiness/health? No, he does not. He may find anything you throw in his cage - food dishes, toys, etc, very attractive though.
4. If neutered will he still hump my arm? He doesn't do it often usually once a week he gets wound-up. He probably won't if neutered.