Dehydration is the concern with gastro I think. And because you've done the right thing getting onto the dr, I'm sure they'll be able to get on top of it.

Big big hugs to you. I would just try and keep him on the breast, hopefully he'll feed, and hopefully the dr arrives soon!

ETA: perhaps a few syringes of water or EBM straight into his mouth might work if he's too lethargic to feed?