How on earth do i even approach this? (not wanting to offend Fil.)

You are not being unreasonable! I would still hire a professional photographer. If your FFIL wants to take photos, great! But at least they will not be the only ones. If he or your FH says anything about, just mention that it would be impossible to have your FFIL take photos and be in the photos.

I was very happy we had a second photographer to do what I have experienced in many weddings. No photobooth. Instead, for a half hour as people arrived, and for almost 1.5 hour during open dancing, he set up and took pictures in the hallway, where he set up lights and a backdrop. Any guests, two families from far with local relatives, pictures of couples, pictures with 1 or more of their friend or family who were in WP, anyone who asked could get a picture. you cannot do group pictures with a photobooth. And individual ones are often terrible. We said no cameras and smartphone shots at all during our ceremony or reception. After the wedding, he made a couple of prints of each photo for us. and sent a file for each who was in a photo, so they could print their own picture. Guests who wanted a file for their brother's family, their sisters, and a third of the 3 sibs together, done.
It was so appreciated. For us each having been away from home at college, working, and the military and grad school, we had nothing but candids of friends and family. After the wedding, he edited and completed editing quickly. Each set of parents got the wedding focused album, and Photo2 had group shots of any friends or family groups we put on album pages, one all his family , and did the same for mine. Getting ready snaps were not done by the pros . So our regular did an hour with FI and I, then each set of parents, and each set parents with us. Took a formal portrait of each wedding party group and such before, so we did notmigss

Sorry ... cocktails. Each photo edited their own work, fast. Both left after 7-12 wedding, though reception was not over til one. ... maybe his dad would like to be a family and friends group photographer, and let you hire a pro for actual wedding party and ceremony and reception candids. This way his dad would not miss any of the ceremony, or dinner and such. Just 1.5 hour group pics during after dinner dancing. The rest of the time, FOG with guests.