There is none. One is a learning disability (I have it) and one is probably because he was abused sometime in his past. He may not remember it because it does get blocked out, but it surfaces in behavior. But in some people it is just a compulsion. Look up Non Verbal Learning Disability in your search engine. You will not see any sexual behavior mentioned. It simply means you are verbally astute, but unable to live up to that skill with your performance. Example: My verbal IQ is 120, which is near superior, and I am very verbal. However, once I get hired at a job and I'm expected to perform up to how I sound, I can't do it and often fail because my performance level IQ is only 85. THAT is a non verbal learning disability.
There is a very low success rate with sexual predators. We have seen every expert available for this and have also talked to social workers who work with these almost-grown predators. First of all, the reoccurance rate is huge, which is why even teens are put on the sexual offenders list for their entire lives. Our ex-son was only thirteen yet he has to register for the rest of his life because of the high probability that he will do it again. Others need to be notified if he is living near them. He has broken that rule and is being charged with a felony, as far as I can tell by his court records.
If this child you are living with does not have to sign up as a sexual offender, it is because your husband has enough money and can afford to get him off. Normally, it would be a no-brainer. At sixteen if you are sexual with a five year old, it is way out of the realm of normal in any sense and he SHOULD have to sign up as a sexual predator. I would want to know if he moved next door to my friendly grandson.
Eventually, boyfriend may also resent YOU because his dear son can't go to any family get togethers and my guess is that after a time he may want to give this son another chance. What if the two of you have a baby girl together? Then this boy is her bonafide brother and I'm not sure you could legally keep him from her if he isn't on the sexual predator list. I think you're asking for trouble, but I do wish you luck. I am puzzled as to how you can feel so warm and fuzzy about a sixteen year old who molested your daughter...he is NOT a young boy...but I hope it all works out. I will spare you and make this my final word.