Kitten vomiting after deworming?!?

I gave two kittens, around 5 weeks old that were both de-wormed at their first vet visit on Monday. They were both deemed healthy. One has been her normal self, eating, playing a lot, etc. The other, hadn't had much of an appetite and has been sleepy, and started throwing up last night, what looked to be the few mLs he had consumed (this one also hasn't weaned, still feeding kmr...started mixing kmr with some extra water and forced it ml by ml to try to prevent dehydration). This morning, he threw up again, however was a whitish foam, which I'm guessing is bc his stomach is empty. I called the vet who recommended to keep giving him fluids, and also mentioned that this was NOT a side effect of deworming. Any advice?? He still doesn't want to take anything in. Force feed? Or will this just induce more vomiting?? Ant advice or experience with this appreciated!