When DH & I married all the wedding party rented tuxes, except my FIL. He weighed about 350 at the time so a tux rental was just not practical. He did wear a sport coat and that was fine with me. I felt the family being together was more important than the photo opportunity. Tell your son that if he insists that his father rent a tux, he can pay for it. Or better yet, ask him which is more important, his father or the tuxedo.