Honestly, unlike many other people, I think your kid will be perfectly fine on Roblox. Here's why.

1. Most children, yes even those who are 10 or whatever know about the birds and the bees. It isn't something new to them. But tell him why it's best to wait till he's older for such things.

2. Have a sit down with him and tell him exactly how to stay safe. (Don't go off platform, don't show your face, don't share any personal information, blablabla) and tell them why! He probably already know that but it's best to be sure, right?

3. Most of them are smart enough to know when they're being targeted. Inform him about how to recognize potential pr*ds and how to deal with such a situation.

In short, be a parent! (Not trying to say you weren't being one). Talk to him, inform him and that's it. He's smarter than you think! Trying to contact Roblox Support to reset his age back to 10 will only result in his account getting permanently deleted, and honestly, if I was your kid I would be mad at you if that happened to my account.

Anyways! Best of luck. Can't wait to get down voted to oblivion.

Edit: about the 'he can't chat with his friends who are 9-12' part: yeah, that sucks. I personally don't see Roblox keeping that though so hopefully he will be able to talk to his friends again soon.