Not a bad seat at all. He will have some extra space to the side of him. The only warning I would give is that he could be one of the last served on the meal service, so I would suggest if he is fussy order a special meal.

I've flown the IAH-LHR route many times in economy, I've never known the load to be too full, so if it is quiet he can lift the arm rests and use the central seats as a bed.