So, um. While your grandpa is having surgery, you'll be having sex. Oh, that's not weird. I say, don't do it. For me, I would want to do it with someone I could have a serious relationship with, and seems like you're both interested in only sex.
Now, that's just me, but I'm sure that might change when I get older. If you think he's hot and you think you won't regret this then... Do it, I suppose.
If you're not sure if you really want to in the first place, you probably shouldn't.
But if it's something you both want, and you're totally comfortable with it, I don't see a problem. Just...do what Lykwoah said.
EDIT: I will say this...if you think it's awkward, it probably isn't a good idea. If sex is awkward, that means you're not comfortable with it, and when it comes to sex it's usually not a good idea to do anything you're not comfortable with, you know?
But, I'm a stranger, and I don't know how you think or feel. If you want it, knock yourself out.