It's very likely that either the meds or the giardia is causing his stomach upset. If he seems OK other than that, I'd just feed him VERY small amounts of chicken breast (shredded into really small pieces) and white rice three or four times a day, rather than a few larger meals. If he isn't keeping that down, pick up a few jars of chicken baby food, and just give him a bit of that mixed with the white rice, again, in very small amounts, or even just a few spoonfuls of baby food to start without the rice. As long as he's drinking water, he won't get dehydrated. If the vomiting continues, I'd check with the vet on Monday. Perhaps he isn't tolerating the giardia med and they might have to change it.
Please keep us posted! It's such a worry when the little guys are sick....