I had a breakthrough with my 3 yr old today so when I saw your post, I had to respond.
It is worth noting that at age 3 they still have baby teeth falling out and adult teeth coming in. I didn't even realize this was a problem until I rode yesterday and my 3 yr old acted like he had no training at all when it came to turning. It was like he regressed and didn't even want to direct rein (several months ago we were working on neck reining and he was starting to get it). So when I rode yesterday I was really discouraged. Then I took off his bridle and saw some pink foam on his bit. Yup, blood.
I never draw blood on my horses, I pride myself in light hands. So I was looking around in his mouth (the best I could with a mouthy 3 yr old) and saw the blood seemed to be coming from one of his top molars.
I then realized he probably has baby caps about ready to fall out and that is where the blood is coming from. Because about a month ago he lost his two top and bottom baby caps on his front teeth and it was the same thing.....bleeding for an unknown reason until I looked at his mouth.
So today I rode him in a hackamore and it was like "wow, he remembers his training after all!" So all that trouble I had the day before was purely because his teeth were bothering him!
So give your guy the benefit of the doubt that it could be his teeth. And besides, your cousins are kids. They are just trying to push your buttons. I almost came to the point of dropping a "friend" one time because she was knocking my horse. I almost told her I would give her up before I gave up my horse but I bit my tongue. And it worked out for the best anyway because I don't even have contact with her anymore. I still have the wonderful horse she was knocking.
So anyway, love your guy and don't let the mean people get to you.
Just because they say something doesn't make it true. And your guy could been having tooth pain at his age.