I have 2 daughters who have had expanders- OUCH! It can take a week or more to "adjust" to the pressure in the jaw, but truthfully, there will be some pain off and on as things shift, and especially after an appointment.
The Ibuprofen is good, but one of my daughters is allergic to it, and we "accidently" discovered that the best painkiller for both girls is MIDOL. Yep! If you can somehow not tell him what it's really for, you may find it takes the pain away faster and better for him, too.
The thing to keep in mind is that expanders aren't worn for very long, and they do get used to the odd feeling after a turn (it registers as "pain", but it's mostly just uncomfortable). Mild achiness and pressure are normal, but if he's experiencing any sharp pain, you want to mention it to his Orthodontist to be on the safe side.