Touching his private parts signals other things as well as a readiness to be toilet trained. It feels good, and almost every child will touch himself or herself from time to time. As is true of all types of behavior, some children seem more prone to want to do this than others.
Judging from the fact that your husband washes your son’s hands when he does this, I am assuming that he is not bowel trained. Take advantage of his signal and put him on the potty, even if he has already filled his diaper. Ask him, “Do you need to go some more?” He might just produce a little bonus for you. If there is any regularity to his movements, it should not require too much of your time to anticipate his need to go. Put him on the potty and sit with him for a few minutes, complimenting him for being a big boy. Bring your 4-month-old along to join the crowd in the bathroom!
Our parenting advice is given as suggestions only. We recommend you also consult your healthcare provider, and urge you to contact them immediately if your question is urgent or about a medical condition.