Hiya Bemused,
This is a tough one, especially if his body is getting used to the lax. Is the upset only (sorry, but we must talk shop!) in the lower intestine (bowels), or is his stomach hurting too?
Anti-B's can be hard on the lining of the tummy, in which case you can use an antacid. But if the Anti-B's are causing him to need the toilet more, surely it would not hurt to even cut the sachet in half to try and see if that helps?
Much luck x