Sluttygran - does your friend have just the one grandchild, or is he favouring this particular grandchild over others? If he is, perhaps you could say something like, "Tell me about ???"(Another grandchild.) Of course, this might just open up another bragging session ... If it does, just change the subject - and keep changing it, until he gets the message!
And if he doesn't, you're going to have to be upfront about it. "That's enough about ???, we talk about him all of the time. Can we just talk about us, for a change?"
When you're out in company, I would just interrupt his bragging with something like, "Yes, he's doing very well, but do tell ??? about our trip to Spain (or wotever!)
There's nothing worse than people going on and on about their children or grandchildren! 3 minutes, fine. 5 minutes, max!