Gransnetters Advice Needed Please

Has anyone any helpful advice please? My now 12-yr old grandson has been touching his penis for some years. Parents and sibling have been trying to stop this, but grandson says "it makes him feel safe". However, today, we were walking in the main street of our town and he was doing it and gave himself several erections. It worries me that if he is out and about on his own and does it, then he might be taken advantage of and get into a situation that at age 12 he would not know how to deal with. My daughter has given me permission to say "not appropriate in public" or similar to g'son, but I don't see them very often. Would really appreciate any ideas on what they could say to him without frightening him about possible predators or paedos. Thanks.