Sender Av, if you tell their parents isn’t that akin to “messirah”? Why does everyone only recognize “messirah” when it involves the other issue? This is also messirah. In all honestly it is NOT your place to reveal to the parents what you saw. Not even to the Rav. However, since you are a young man and not “old” like their parents, maybe you could be a mashpiah on such kids who sometimes need a “big brother” or someone to look up to. That is if you really care and want to get involved. Just start by saying hi to the kid when you see him and ask how he is doing. Don’t immediately zoom in and speak to him about being mechalel Shabbos. You can offer to play basketball with him, or ask him if he can help you figure something out on your phone or computer. After all he does live on the block and you do know him, right? If you strike up small inconsequential conversations you can build up to being friends and have a positive influence on him.
You don’t know what his issues are. But maybe because you are older than he is but yet still NOT in his parents category he might ope up to you and allow you in.