As salamu alaykum,
Children at this age and younger do discover that touching oneself on their private parts feels good. Even babies are reported to touch themselves often by accident but find it pleasurable none the less. And guess what, it usually occurs around potty training time.

EverDayHealth (1) states “Your child’s new love affair with her genitals is fueled by age-appropriate curiosity and is as innocent as her exploration of her fingers and toes. You may notice this behavior around the time you switch her to training pants (accessibility is key) or when you begin potty-training (her private parts have now become the focus of a lot of public attention)”. So, what your son is going through is normal. It is an age appropriate curiosity and response. While it is now a behavior that is undesirable as he is doing it in public, or while engaged in an activity, rest assured that he is not doing it purposefully as some older children who know better may. He is merely reacting on a primitive response from a pleasurable stimuli.

I would kindly suggest that when he does touch himself that you do not yell at him, or tell him he is bad. Rather pull his hand away and divert him to another activity such as a toy, a game or other distraction. Ask him if he needs to go to the bathroom and take him. Often times a child will touch themselves if they have to use the restroom. If this is not the case, if you take him to the restroom when he is touching himself, he may associate the bathroom with self touching. I don’t know how old your child is, but explain to him in terms he can understand that his body part is private and not to be touched in public. Additionally, when you divert him from the activity and he stops, praise him for his good social behavior. It will take repetition and patience on your part, but in time these behaviors should stop.
While these are normal reactions on his part and a natural response concerning growth and development, they are behaviors which should be diverted but not punished or made a big deal out of. The more emphasis and focus you put on it, the more he may do it as he is getting a big reaction. If you yell or punish him or tell him he is bad, he may develop low self esteem about his body, sexuality and functioning when he is older.

A fine balance is needed to stop these behaviors but with patience and consistency it can be done! We Wish you the best.