My father is a marine .... Should he wear his dress uniform?

I guess that's up to you and your FH. Does your FH want to be the only one in the uniform or does he not care? Do you have a particular reason you want your father to wear his? They are pretty uncomfortable to wear for the entirety of the night so keep that in mind if you'd rather your father just be comfortable. My FH wore his dress blues to his aunt's wedding (her choice) and he was a bit miserable and felt on edge about not disgracing the uniform (it was pretty early on in his Marine life). But the idea in my head of having both him and your FH in the uniform while your father walks you down the aisle sounds like it'd be a beautiful idea, but I'd recommend allowing him to change into something more comfortable if he wanted.

My FH is currently in the military and his father is retired military, and neither wanted to wear their dress uniform. They both find them to be pretty uncomfortable lol.

I think its up to personal preference.