Is this something new? I mean, I assume your FIL visited you formerly while his wife was alive.
I would try the tactful approach first and ask him if there are any foods he finds hard to digest and you are wondering whether something you serve, or whatever you fry food in is causing him to belch so much after eating.
If it is indigestion pure and simple, you should be able to avoid whatever causes it if you know what that is.
Do you and your husband eat faster than your FIL is accustomed to doing? Trying to keep up with fast eaters could well cause belching. So could ill-fitting dentures.
If he says something like, oh I always belch after meals, then you will either have to put up with it, or say that you would prefer him to leave the table before doing so.