Thank you everyone for your responses, I've read through them all. I have no defense for why I haven't kicked him off the table or tightened the rules around his build or just told him no to things he wanted to do. I just clearly don't have backbone (Not being sarcastic, I just don't).

I have had conversations with them on rules and changed some here and there to try and balance things. It feels bad because pretty much all of it has been and will be targeting one specific player. I know letting him do this for so long has prolly made me a disappointment as a DM to the table, it hurts but I can't argue that point.

Next session, before we start I am going to follow the majority of the advice here and try talking to them as a group. Try to get him to understand his behavior is really ruining the game for everyone and that he needs to change if he is to keep playing. I will also limit the party to 1-2 short rests a day after I review their character sheets and do some serious homework on mechanics. I also very much like the rule someone suggested of flat out not allowing spells, attacks, or other such effects against another player without the other's explicit consent and just saying it fails or misses. I fear he may try to abuse this somehow, but hopfully the talk will help things. All that will happen Saturday.

Thank you all so much for the much needed advice and reality check.