It depends on how tight with him you are. If you're cool, give him a call up and offer to take him to lunch. If it's not a good close relationship yet, swing by his house when you know he's home and your fiancee isn't (you can think of something).
A good way to establish a solid relationship with the FIL. Lucky for me, Mrs Sizzle has no FIL, so I dodged that awkward moment.
