its normal its completely normal
occasional experimenting and curiosity is normal, but if it happens frequently, well that is a different story. 10 is the age where kids start thinking about things sexual, even if they don't know what they mean. your son needs to understand that what he is doing is technically illegal. you don't want to shame him about anything sexual, for he may end up rebelling against you if you flat out tell him not to do it, but again it is illegal and both him and the person he is doing it with could get into a lot of trouble.
beyond natural curiosity he needs to learn to satisfy any urges he has on his own. he can't be dependent on others to fulfill these desires, especially at such a young age.
some would say that it is normal however I think the ages matter an older child touching a younger one is not a good thing. I would tell him and the others /parents that maybe finding some other hobbies would be better How do th other parents feel ?.