There's nothing special you need to say, just tell them your honest feelings. Tell your mom and dad in a respectful and loving way just what you told us, that you would like all of you to spend more time together as a family. Even the busiest working parent will take extra strides to carve out an hour here and there if they know you really want it and it comes from the heart, because they love you and want what's best for the family, too. And there's plenty of things you can do together if you use a little imagination, even with one parent being disabled. As others have suggested, video games and board games are fun, heck, as long as your dad has the use of his arms, you could even go bowling, play horseshoes, all sorts of things.
So you do what you can with him. Hes your dad. It would probably thrill his heart if you asked him to play a card game or board game or something. If his arms are affected there are ways to addapt games. Card holders, ways to use his mouth, hand braces, etc.
Depending on his disability,if he has a wheelchair go for a walk with him or just spend some time with him. Hug him every chance you get and tell him you love him. And tell him thanks from the rest of us for his service to our country.
If your Dad is disabled, he's probably missing a lot that he used to have. He may even feel guilty for not being able to do things with you that he could if he wasn't disabled. Your mom is probably stressed now that she is the provider of the family. So basically all of you are feeling a lot. I would talk to both and say how much it would help all if you if you could spend a little family time every week. Good Luck. Don't be discouraged.
Some things are best left unsaid. I'm sure your dad isn't thrilled about being disabled either, he probably shares your same feelings about not being able to do certain things with his family either, if you say this to you risk making your father feel like he's a burden because of his disability. Instead of focusing on all you can't do try being happy that you still have him around to do anything with. There's countless families out there who aren't as lucky as you, tons of kids who's parents didnt or won't make it home.
Ask any of us who have been to Iraq, Afghanistan ect and came home unable to do many things we did before. It kills us not to be able to do things with our family and friends that most others can do every day. It can really make you feel like a horrible parent. Find something interesting that all of you can do together even if it's only for a few minutes at a time here and there.