This sounds like the reverse of the situations in my dad’s former parish. His former pastor has actively solicited the communion-age students from a local private non-denominational school (kids are Catholic) and is providing them with preparation and reception of the sacrament. These kids do have parishes of their own, but, for whatever reasons, he has brought them to the parish. Mind you, the parish is several miles away from the school and the kids do not even live in the downtown area (where the parish is located). Whether or not this is allowed is another question altogether; but, it just doesn’t seem right.
Regarding your son’s wish to receive his First Holy Communion with his friends, I can understand his concern. When I received my first Holy Communion 32 years ago, I wasn’t able to be with my classmates from Catholic school (at the time, the nuns told us to go to our parishes). I didn’t know any of the kids, but, that in the end, as my second grade teacher (who was a nun) explained to me, it didn’t really matter. All that mattered was that I received Jesus for the first time. It doesn’t matter if you know one child or the whole class. All that is important is that this is the first time that Jesus makes physical contact with you. That’s what counts.
I hope this helps.