You are all so wonderful.
DP is determined that we go with him tomorrow, as he can't face it on his own, so in the spirit of lalsy's "just do what he wants", that's what we'll do.
I think I'll tell DS what's happening before we go.
And as for stars, I've already told him that he was a star in the sky before he was born, and that the cat is now a star, so it works perfectly.
It's a funny age - old enough to be affected by it, but not old enough to really understand.
And stropps - I can imagine in your circs, the dog's death timing didn't exactly ease matters. But I suppose if a guinea pig dies a convenient year or so before a grandparent, it does help introduce the concept of mortality.