I'm a father of 3 boys and raised my lil broter. Boys play with their parts all the time. It is normal. It doesn't have to be shown to them or they don't have to be copying. It is a normal boy thing, it is there, they are going to play with it. But, yes u can try to distract him, but doesn't always work. (Iv tried) I also have a 4 year old with autism, he constantly plays with his penis as do his brothers. Boys will be boys! U may not like it but boys do stuff like that. It is second nature for them to pull on it. As long as he isn't doing it in public, I wouldn't sweat it. Hope this helps. If u have any other questions feel free to message me.
Distraction should help when you see him doing it take his mind away from it, get him busy, playing doing something he likes, bordom will make the habit worse and some of it is normal in children ,is he at school ,day care , does he do it when he is in that enviroment. . Any older siblings he may be copying ?.