I have no idea about the testicles, these 3 sheep are our first foray into sheepdom and that's above my current paygrade. I'm hesitant to pay a vet to check given the likely outcome.
No real need for a vet. If there is a retained testicle that slipped out of the band, flip the wether over and feel in the groin. The testicle will be a golf ball or larger lump in the groin under the skin in the area where his testicles would be.
If the testicle was never descended, then it may have been retained in the body cavity and you won't be able to feel it. However, in that instance the only option will be to take the wether to the vet for a surgical castration which will be expensive.
Even wethers can get territorial and pushy. Flock animals determine their own positions within the flock hierarchy. Until now he has probably been lower ranked than most of the ewes and your children. As he has aged and increased in size, he is looking to change his flock position. Since he is large and has decided to dominate your 6-year-old, you need to decide if you want to fence him off from the children or not. Since both your wife and older boy have noticed strange behavior with them, that is a problem. It is possible that he is sizing them up to see if he can dominate them.
His good behavior with you simply means that he has accepted you as the dominant ram in the flock. He is currently trying to change his position within the flock. An adult sheep can break a man's leg if they butt hard enough. Since he has no value other than as a pet,
and with his current behavior he is not a reliable pet, I would say his time on the farm is done.
Frankly, I would just eat him and start over with another lamb. Or take him to the auction and get paid for him. I would not take chances with my family. No matter how sweet and lovable he was as a lamb, he is a flock animal and his behavior will be determined by that.