Since he won't need a park ticket, he shouldn't need his own express pass at his age, but if he did, it shouldn't be a problem to get one from the front desk at the RPR.
There are definitly attractions that he would be able to ride (similar to the one's at disney) Such as 1 fish, 2 fish (same as Dumbo), the Caro-seussal in seuss landing at Islands of Adventure, there's also a barney show/play area in Universal Studios (as well as fivel and curious george areas, but they seem aimed at older kids). Hmm..thats all thats coming to mind now, but I'm sure there are others...