Father of the groom attire

This is a conversation with your FH, but general rules are, the fathers should at least match the level of formality of the groomsmen. That means if the groomsmen are wearing tuxes, the fathers should wear tuxes as well. ... He can also choose attire in the same color that features a different cut or accenting than the bridal party.

It's a should or must factor. Only you know your future in-laws, maybe he's just not a suit person

I don't think i'd want to force them to wear something they don't want to but you can try to make suggestions like how about a vest over the button up shirt or a suit jacket at least

I agree with Melle. You might also want to say something about how your dad is wearing a full suit, but you're open to hearing options for your future father in law in order to make sure everyone is comfortable. In the end, I would say them being comfortable would be more important because you don't want a look of discomfort on his face throughout pictures. If it's not who he is, it's going to make him uncomfortable. Best of luck!

Suggest that he can wear his boots and hats, along with the suit and he can change after the pictures?

You do what you and your FH are comfortable with, perhaps you can have a compromise as mentioned above - the hat and boots with a suit jacket or maybe something else - a funky belt buckle maybe? Have you talked to your FH about how you feel? My FH and groomsmen will be in tuxes and my Dad and mother's partner will be in navy suits. It is what everyone has and can afford, and it doesn't really matter to me, as long as everyone is there and comfortable. Good luck and have a wonderful day!