Glad you got the hub fixed to your satisfaction. There are many items that can be fixed on karts but some that you are better off replacing. Experience and help from others at your track will teach you which is which but there is a “bible” as it were for race car fastener application. Get this book and read it, then you will know for yourself.
As for your other questions you should make an appointment with the tech steward at your local track on a practice night and go over your son’s kart with them. Rules vary from club to club so a few minutes spent with the person responsible for tech will save you headaches and heartaches come race day.
I am assuming you’re new to the sport, if not then I apologize but hopefully the info will be useful for someone else.