I don't know what their relationship is like or what kind of dad he is, so I'll share from my situation in case it's of any value.
My son used to get discouraged about what to give his dad because "his dad didn't need anything and already had way too much junk" according to my son (which was pretty true). My son kept bugging me to get him lego sets. At first, I thought that it was just my son really wanting a present for himself, but then as I thought about it some more, it didn't seem like such a bad idea. My ex is an engineer, so I decided to find things that could be projects for them to do together, that my ex would enjoy, and where it could be a bonding experience for the two of them. I've gotten him various things like circuit building kits and other things to build.
Perhaps in time we'll find more personal things to give his dad, but for now, that works, and saves my son from experiencing his dad discarding a gift that he doesn't like or need.
The point was to find an activity that my ex would enjoy, and enjoy teaching, and to make that into something they could have fun with together.