What could this be?

We are going to the dr in the morning anyway but my little man (21m x28w premmie) has been throwing up and pooping runny poo for four days..
I would have took him in sooner but he hasn't had a temp and has been keeping some fluids down.. He probably throws up 1/2 of what he eats sometimes more and is pooping like water about 7-8 times a day..
He is still drinking water..
Is there anything a dr will do?
I'm really worried because he is super tiny and is classified as failure to thrive anyway.. He is 7.9kg and honestly looks even skinnier to me since getting sick..
Just wl drying what it could be? Or what can be done to help him?

You're right to take him to the doctor. Sounds like a virus his little body might not be able to fight off as well as his brother. I don't know what they'll do but good luck. I wish you all the best and a quick recovery.

It sounds to me like he has a stomach bug, but do keep us updated on what the doctor says.