If walking away doesn't work then try a time out. Tell him no quietly, take him by the collar and put him in his crate, or a small room like the bathroom until he calms down.

I think dogs are smart enought to figure out what kinds of games they can play with different people, so if everyone is doing the same thing with him except for your grandpa Jasper will learn that biting everyone else is not okay and biting your grandpa is.

On other thing you could try is the award a prize method. When he bites you, you cheerfully award him with a prize, such as cutting his nails, or a long down stay, or a trip to the outdoors, or whatever he may not enjoy too much.

Hope you find something that works. Good luck.