It's very nice you're willing to help your dad in this effort. You're correct, he needs larger portions than you. Other than that, he should be eating the same thing that you're eating. You don't indicate if he lives with you or who is going to prepare his meals. It would be easier if you're going to do it.
You didn't mention either if he has any health issues, anyway, he'll need to eat something from the approved list every 2 hours at first to keep his sugar level.
He'll lose weight faster than you do too, which it's good for him.
You didn't mention either his age, but at any age, you need to encourage him to exercise at least 30 minutes every day, even if it's just a small walk.The weather is getting better and he can go for a walk outside almost at any time of the day.
For meal plans, whatever you're eating would work for him, pick the things he and you like that are in the list and make your menu. He will be more agreeable to eat the stuff if he's familiar with it, rather than creating fancy menus with foods he has rarely or never eating before. Remember, this is a way to eat healthy, losing weight is a side effect, so don't make his menus looking like a diet either, look for recipes in our recipe section and prepare for you and him some yummy meals including some desserts.
good luck and let us know his progress!
Ami in OH