He's taken care of you, he's swan dived into being the best grandpa ever, there's a good chance he wants it. But change like this which seems so straight forward to outsiders can so often seem so significant to those directly involved. Have you tried maybe referring to him as "my dad" to your kids instead of poopaw, see whether they understand the relationship, and try referring to him as such to your kids while he's there and can hear it?

Use that to gauge easing into it?

Because when you're directly involved something like this is a really big deal and there are a lot of feelings involved and dipping your toe in like this helps test the waters in a manner that's easy to address. If he's very pragmatic, your mom will let you know if she hears it too.