My kid acts out horribly with sitter only - he's the "problem kid"

Okay, it's very tough for me to post this as I'm starting to near my last resort list.Background: My son, currently aged 3.5 years, has been under the care of an in-home (as in her own home) sitter since being about 5 weeks old. He has never had another sitter.She's a nice and sweet older woman, possibly late 50s, early 60s. At home, and literally anywhere else, our child is fairly well-mannered for a 3 year old. I'll admit he will give more of an attitude to those that aren't me (his father). He even listens to mom less than he does to me, but either way, she's definitely the next-in-line point of authority, so end of the day she wins the battle. He doesn't have other kids around that he plays with regularly and at the sitter, there's really no order in place. There's no scheduled activities beyond eating. They just kind of leave the TV on and let the kids play all day. No routine.Also, there's about 13 kids in the house, total - all aged between newborn to about 6 or 7.Now: For the past several months we've heard from our sitter off and on that my son has been acting out here and there. At first it was him throwing tantrums over not wanting to change clothes...or take a bath, or just something of that nature that didn't involve other kids. As of lately, we've been hearing reports from her that he has been violent with other children. He'll bite them; slap them; pull their hair.JUST today, I also heard from the sitter that she had been withholding information from me, regarding his behavior. It turns out that my son has been completely beyond reason with her. He'll also slam doors, bang on them, tell her to shut up, cover his ears and just act out. He has no respect for her whatsoever. He also gets jealous when she treats children well. He always wants to come first. He wants to be served first. If one child is getting a special treat, he wants the exact same thing.This has all come in waves and I am so stressed. I feel at a complete loss with my son so much so that it makes me shake. My wife wants to give him the benefit of the doubt and keeps making excuses because she feels we aren't getting the whole story. She also feels if anything we need to find a new sitter, someone that actually has routines and authority in place. I feel like I want to skip ahead and put him into behavior therapy because I don't think he should be acting out like this regardless of the environment - we just can't find a middle ground.Can someone, please, lend some insight into similar situations you've had and how you handled them. We feel defeated and we're worried about him starting school soon and this happening while he's there or under the care of others.EDIT: Added a few more details.EDIT 2: The sitter DOES have one other helper, but that's still probably too much for both of them, considering some of the children are infants. via /r/Parenting http://ift.tt/2g8va7e