I would say vet, as soon as possible. If he is not feeling well and not taking in fluids then he can become dehydrated. When Lilly had gastro when I picked her up at the shelter the vet gave her a shot for nausea so that she could take fluids.
Oh dear. I am sorry he is still sick. When last did he eat? Is he vomiting bile? Does he have a tempreature? Is the vomiting violent? What colour are his gums?