Ya, maybe offer him cheap, and if your intentions are to keep it forever and pass it down, let him know that.
Look up the bluebook, show him the high, and offer the mid or low.
Entice him to dicker with you by telling him you know what he paid for it, lol. If you really want honest opinions of what it could be worth we would need more pics and info. It looks like it could be a verry nice, low mile original. A collector might pay a high price for that one.
But you might just be lookin to take over the watch, keep it in the family. Or, you might be lookin for a bargain or a flip.?
When you own a bike like that, sometimes, you're afraid to ride it.
Buy it from your father-in-law, you might have to explain every scratch.