Not sure where to post this subject: 6 year old son plays with himself, when needing a wee

Ok so I never have really posted on these things but I'm struggling to find advice when I googled it.

My near 6 year old boy for the last 2 years has had this habit.

I know it's normal for children to explore themselves

The issue is he only likes to play with himself when he need a wee. He always sits on the floor in this room or behind a curtain he says when he needs to go for wee it gives him a tickly feeling. We (DH and I) have told him he needs to use the loo before doing this first but this doesn't work. I even said can you do it at least while on the toilet. He agrees but never does what I ask. So I'm very frustrated. He wets himself everytime he plays with himself and often the carpet (it's always a sit down on this bum position) We have tried many things to help him even make him stop. Time outs, no outing, reward chart, privileges given or taken. Im at my wit's end and I don't know if I should seek professional help. Advice would be appreciated. I tried to talk to my mum and mum-in-law and I think they just think he is just too busy to use the loo but I know it's not that . It's a habit because he likes the feeling. But he says he wants to stop because he doesn't like wetting himself. I thought I might send him to the loo every hour but we are struggling to find anything that works.