I imagine the green was bile rather than phlegm. If I were you, I'd just give him diluted juice or plain water. Try to make sure the water is not freezing cold, my dd always voms the lot back up if it is, which is a PITA as she won't drink warm(ish) water.
Thanks kreecher, sorry I didn't see this earlier. He was sick once more at about 8.30 but thankfully nothing since then - just giving water him water and some toast for lunch, he has perked up - hopefully that's that. I always get scared it's a bug and we'll all get it and have days of misery.