Most HP fans will have a house they relate to, even adults. Think about the houses your DCs were in at school. In Harry Potter the houses kind of associate with the individuals personality, that’s how children are placed in them. My DCs have very specific houses they have related to so anything we buy HP related are in their colours. Kind of like having football teams and their colours.
If he still likes Lego, any HP Lego would be good. Lots of adults still love Lego, we have a number of sets, officially they are for our DCs but DH & I (late 40s) love them.
You could get art work, kitchenware, clothes, you will probably find lots of HP Christmas jumpers, cosy socks in the run up to Christmas. HP has a lot of confectionery which you will find in lots of places.
Boots do HP themed gifts.
I would try and find out what he has and if he has a specific house, just ask him, what’s your house so I can get you something you want.
I echo the studio idea for tickets or look for other Harry Potter experiences.