I’m very glad you wrote. Your wife’s response to her son is not unusual even though it is not helpful to her son or good for your marriage. My guess is that she feels sorry for her son and perhaps even carries some guilt about his disability. In response, she is enabling his antisocial behavior.
You and your wife both need help and support while you navigate better ways to manage this young man. Seek out a counselor who has experience working with families with an intellectually disabled member. Your marriage and his future depend on it.
If he meets the criteria for intellectual disability in your state, there may be state services including residential placements. Contact your state office for Intellectual Disability services and ask to speak to a social worker who can give you some guidance about what kinds of help are available.
I wish you well.