Maybe try a black bean burger. Top it with some vegetarian chili and he’ll never miss the “meat” I bet. My brother is a full fledged meat eater and he liked the Lightlife Backyard Grillers. They sell them at Kroger around me. I haven’t tried them because they are not vegan. It can make for a better transition burger to a 100% vegan lifestyle. Also try the Masala Burgers from Trader Joe’s if you can get to one, very yummy and 100% vegan.
Maybe also check out the documentary “Forks Over Knives”. It can offer some help on eliminating his high blood pressure and diabetes through diet alone. It’s a big leap, giving up meat, but it gets easier the longer you go without it. Oh and potatoes are great! So you guys can be full-fledge no meat and potatoes people from now on. Good luck on everything! 🙂