I hope he's feeling better soon. If he hasn't perked up when he wakes up I'd get the home Dr service out for another opinion, and they may be able to give him something for the nausea.
My DS was brushed off by a stupid GP when he had tertible gastro once. He ended up in hospital on an IV later that day. I was so angry at the GP, but so glad that I got a second opinion and they told me to take him to hospy.
Definitely try hydralite. We use a panadol syringe to just give him small amounts at a time and leave 20 mins in between each to give it time to settle in his tummy. (DS has had gastro like 6 times!!) Also, stay clear of dairy as having a tummy upset can cause temporary lactose intolerance and giving milk, etc can make it worse.
ETA - I'm not sure how old your DS is?? If he's under 2-3yo I'd bypass a second opinion and head up to the ER too.