It’s great that he wants to go!
Definitely send him off, and talk to him a bit about some of the potential mood issues so he’s aware of them, but I would keep this pretty bare bones. You don’t have to harp on examples but simply set the scene that because of his age, your dad can get frustrated or tired quickly. Set some expectations about how you want your son to respond, along with some actual helpful tips (“give Granddad some time alone if he starts to seem overly cranky” or “work on a project for short periods of time, even if it takes a few more days to finish” or, like with toddlers, “wrap up an activity while everyone is still in a good mood, don’t wait for him to overextend himself.”) so that your son can anticipate how he (your son) will behave in these situations.
I think it’s sweet that you only want him to have “good” memories of his grandfather, but ultimately I think it’s more important that he has “real” memories of this grandfather, and if you put some responsibilities on your son to take the role as a grown up (or at least, more grown up) member of your family – like it sounds like he would really be HELPING your parents, not “pretending to help” like a little kid would – he could also remember this experience as a time when you trusted him to be mature and independent.