T.H.
I think you could probably transform a lot of your regular dishes into vegan ones. Margarine is vegan so you could try substituting that for butter in things like mashed potatoes and sweet potato dishes, you could also use soy milk instead of cow's milk. I don't think marshmallows are vegan, pretty sure they have gelatin in them so if you guys usually put those on your sweets maybe just do it on half, or make a small dish just for him. Green vegetables you can probably just add seasonings and cook it regularly and then have butter available if someone wants to add that on. If you usually do green bean casserole then just keep out a section of beans just for him and steam them with salt, pepper, maybe even some red pepper flakes or something. Sourdough bread is vegan so maybe make (or buy) sourdough loaves or rolls for dinner instead of traditional ones.
I know it will be a tad more work, but honestly, if you just sort of portion out a little bit of each thing before adding milk, cream, butter, etc. that is just for him it will be easy to make it tasty and vegan. Or, like I said, just make everything with margarine and soy milk, you probably won't notice a ton of difference.
As far as meat, well he may be on his own for that one! They do sell Tofurkey, but I'm not sure how big it is or how to cook it. If you're feeling super adventurous then maybe hit Whole Foods and check it out, I'm sure it's not that hard...you could also probably buy a slice of vegan pumpkin pie (or another dessert) just for him.
My SIL went vegan for health reasons and my other in laws have various allergies so I have lots of practice in making separate dishes! Plus I have some very good friends who are vegetarian and vegan. I'm no expert but if you have any other questions I might be able to help you!