Has he got his own copies of the Harry Potter books/dvd’s?
There’s also the spin-off franchise, Fantastic Beasts and Where To Find Them out now. There’s a DVD and screenplay for that if he hasn’t got them already.
Harry Potter Studio Tour Tickets could be an idea, even if he’s been once it’s quite big so can easily go again and have a good time.
If he is really into Harry Potter you could get a Geekgear subscription box, they’re about £25 and you get a selection of HP themed things. Or there is a wearable box where you get 4 t-shirts for around the same price.
Does he need anything for his work space, like a new office chair?Fitness/sleep tracker?
Small gift ideas; overnight bag, bottle of alcohol – whiskey/vodka/spirit, sweets – liqueurs or childhood favourites – good value jars of sweets at wilko, shirts, novelty tshirt, pyjamas, aftershave, small planner 2018 for work, men’s razor gift set, wallet, any favourite skincare/hair products?, earphones/headphones, USB memory stick, alarm clock, nice mug, electric toothbrush, Amazon echo dot (some sort of speaker thing)