it he does 2 days, believe it or not the 2 plus 3 days ticket which is sold on the internet is your best deal. There is no 2 day/one park ticket available except through timeshares.
Just looked on the UO website and there was a one park/one day ticket, that if purchased online would give you 4 extra days. I had not seen it before, but this sounds perfect for him.
It's been there for awhile but please note it can only be used in one park for 5 days. Whichever park you enter, is the only park you can visit for 5 days. If your son just wants IOA then that would work for him. I forgot about that ticket. It is only available online though so purcxhase before you go.