I found this site which has a "seat belt lock" which is a plastic case you can put around the seat belt buckle its ***

I have had this same problem with my 6 year old bipolar child. His moods have been better on lithium but he has been known in the past to unclip his seat belt, and his brother's also hitting me while I am driving. I know the fear. I hope your child can get some medicine which will help.