Let me start this off by saying DS has always been quite a healthy child and has never come down with much more than an occasional cold, and I'm ashamed to admit I really don't have much knowledge or experience of handling any other illnesses and don't know what to do.
4am Monday morning DS (3) came into our room complaining about his tummy hurting and ended up being sick twice that morning and also had a runny poo.
Kept him on plain foods and lots of water throughout the day, in the morning he also managed to sneak some blueberries (his absolute favourite) out of the fridge when I was upstairs changing the baby and threw those back up 30mins later, that was around 9am, he was fine for the rest of the day plenty of fluids, playing and jumping about and towards the end had some rice as I was concerned about him not really eating and he kept it down fine.
DH attributed it to heat stroke as the day before we were out and about quite a lot and he did spend a lot of time in the sun.
He slept through the night with no issues and I made the rookie mistake of letting him have his usual cup of milk in the morning which he threw back up within the hour 
He's completely fine otherwise and isn't in any pain or discomfort, he's happily playing and is very lively doesn't have a temperature or any other symptoms apart from just not being able to hold food down.
Any insight on what's wrong with him and what I should do? Also should I keep him off nursery tomorrow?