WOW, although it is surprising and shocking to us what they choose to participate in and connect to, it is just natural and normal to them. You would be surprised how many OTD non shomer Shabbos still fast on Tisha b’av. “Of course, why did you ask????” To them we are asking foolish questions so WE MuST learn to STOP asking these questions. Just take two deep breaths and hold back. It is better not to ask just watch, listen and learn. Amazing isn’t it that when we think they are so disconnected they are still connected in some ways. They really choose NOT to break all ties. And who are we to question that??? Why call them hypocrites when it was our own hypocritical frum JEWS that shoved them off the derech to begin with? They are kids who are challenged and confused. Lets appreciate that they didn’t completely turn their backs on our Chagim and traditions. Think about it. They could have chosen to just go to a club or party that had nothing to do with the chag! Better that they chooseto connect with other Jews. When they do so there is always a possibility that they will meet the right sheliach. Yes even at a chagigah.
So go ahead and prepare for Peasach and don’t be afraid to ask him for help. The worst that can happen is he says no. The best is he helps in some way and he feels productive. It sounds from your post that he went back to school.