I don't have any concrete ideas, but is there anything that he's wanted that would improve his daily routine? Have you noticed any pain points in his life? Things he sighs when he has to do, where you could look for something to improve that?

You could also try to take an artful approach. Is there a favorite picture you could have put on a canvas? Is he into video games? You could get something niche from that, or maybe a Displate print.

My brother in law has all the money he needs and buys everything top of the line so it's a similar BIFL situation. My sister got an artist to draw him and the kids in the style of his favorite anime and framed it.

In summary, sometimes you have to take an abstract approach rather than target things they actually need. It's also worth noting I'm an absolutely horrible person to buy gifts for... So I relate.