His knee won't touch the seat in front, but it'd still be a miserable experience for a long flight. But as someone who is 6"2 economy plus is totally worth it if you can't splurge for business class.
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.