I agree the younger the better. As long as both testicles are down, I wouldn't do surgery of any kind to find the "hidden" one until the horse was 3 + years old. If they're both down though I say get him gelded and then you won't have to worry about it. It is still early enough in the year the flies won't be too bad for a couple more weeks and he should be pretty healed up by then.
Oops I just reread your post so if he's only 4 weeks now you may have to wait until the flies leave this fall, but he still shouldn't be showing any stud attitude by then. In answer to your other question, yes they change when you have them gelded, if they are old enough to know they are a stud before gelding. You probably won't notice any change on yours if you have him done young, but an older stallion does show a considerable change in attitude when he realizes he is no longer a stallion.