I misread your original post, if all your son cares is leg room, then I'd look at booking an exit row vs premium economy, the reason being price of premium economy usually comes at a hefty premium, where as economy plus is more reasonable. But if shoulder room is also important then premium economy it is. Leg rest are useless for tall people in my opinion. If you have the exact itinerary, then I'm sure the knowledgeable fters here can give their advice on alternative airlines.
I thought PE only had extra leg room. Are the seats wider too? The seat chart still shows the same seating configuration as economy on seat guru.
Edit : NM. Apparently there are 2 versions of the flight. So I guess at the time of booking I should make sure it has PE on all legs of the trip.