OK, I do think this is slightly different, not the same as playing pranks on those outside the group.
They are a group, together they like doing this, they all find it funny.
That's fine, and it is a particular sense of humour and it is fine when it is contained within the group.
April Fools is basically pranking, and I used to do April Fools on my class as a teacher, gentle, and harmless ones, where they all laughed when they realised. (things like the date on theboard says 32nd March) No one person/child is the brunt of it.
The spaghetti tree on TV in the 1970s was and still is one of the best April Fools, and it is funny. Genuinely funny, but the key was that there was no victim.
So there is a place for them, but the problem is that is needs to be kept within that group, and kept in a place where it is still fun for all concerned. That can be a very fine line, and 10 year olds are not noted for their subtlety in negotiating those lines.
So I think it is time for a conversation, and for him and his friends to take note of the boundaries for this.