1. Tell him straight. "A chameleon is not a person. I know you really want to be friends with him- and that's great- but you could really hurt him by handling him right now. We are both going to leave him in the cage until he settles in. It will only be a few weeks, and then we can bring him out again" is pretty clear, no matter his age. Let him know that you will have to follow the rules, too, to make it fair! Make sure he understands what is expected of him, that you have to follow the rules too, and that there are dire consequences for not following them. Ask him how he feels about that, and try to address any concerns or issues he may have in a way that shows him that you aren't just being mean or bossy, but that you are honestly concerned for the health of the chameleon.
2. Get your parent(s) or guardian(s) involved. Let them know the situation, and ask them to talk to your brother as well. Get their authority on your side so your brother knows there are additional consequences other than your displeasure if he goes against the new "cham house rules". A family discussion would be a good idea, so that everyone is on the same page.
3. If your brother is interested in chameleons, that's great! Maybe you and he can do some internet research together (with your parents/guardians permission) about chameleons. Let him explore his interest with you. Show him some articles about how sensitive chameleons are, and how important it is to give them time to settle in after a big change. He's old enough to read the articles himself, and to understand them. Sharing interests with your siblings is awesome: I'd love it if one of my sisters was interested in chams!
3. Since he seems interested on doing chameleon-things, let him join in: give him a job to do. Maybe he can do the afternoon misting, or maybe he can handfeed the first 2-3 insects of the day. Let him interact with the chameleon in a safe fashion, so that he feels included but your chameleon is given the space he needs to adjust.
He's not very patient but this could be a good learning experience for him.
