The main thing is to find out what he already has, because there's no point giving him a gift if I buy something that won't work for him. But if you're just guessing, the best options are little things that actually make life easier for a vinyl lover: a quality brush for cleaning records (not that cheap anti-static one, but a proper one with soft bristles), special nitrile gloves for handling records, or this cool gadget - a vinyl needle to USB converter. If his turntable is already old, this stuff really helps.
If you have a bigger budget, think about the sound. Previous answers suggest upgrading speakers - that's true, but you need to understand that people buy speakers themselves because they're picky about them. But a phono preamp - that's something collectors often forget about, and it's the kind of thing that genuinely improves sound quality if the turntable is connected to regular speakers without a built-in preamp.
The third option is pretty practical: a good case or organizer for storing records. Sounds boring, but if he already has a hundred records, he's definitely tired of hunting for them. There are vertical shelves made specifically for vinyl, and if he's already drowning in previous gifts, a proper storage solution is basically the perfect present.