I think you've covered everything Helen (except maybe the worming as you've said) that's why I like the injections for worming piglets - you know they've definately had the right dose and they are still small enough to hold when jagging
It's possible he's eaten something that's not agreeing with him, or just gorged himself on something. Do you have sheep nearby ? See some of my recent posts on clostridium & lambivac if you do.
Other than that, if he's drinking and moving I'd give it the 24 hours and phone the vet on Monday.
HTH
Karen x
Bet they're getting pretty big now