Problem is with flat lemonade - most fizzy drinks are now sugar free so it doesn't work in the same way and sweeteners can make things worse.
OP - it's a bit late now to try this , but you could if he's still vomiting all fluids in the morning.
You can use water or diarolyte. You can try dilute squash but most are sugar free these days.
1st hour 5mls fluid - 1 teaspoon or 5ml syringe every 5 minutes - no more than this. It's enough to be absorbed by the mouth's mucous membranes but should not trigger a vomiting reflex.
2nd hr 10mls fluid - every 5 minutes again.
3rd hr 20mls fluid - you can go down to every 10 minutes at this point
If he vomits, go back a step. Once you've gone past 3hrs with no vomiting, let him drink when he feels thirsty.
Write down when he pees, even if it's a small amount.
In terms of food - leave it until the vomiting has stopped.
If it's just diarrhoea then he can eat whatever he wants. I wouldn't push him - it's best to follow his lead, if he's hungry it's worth trying something.
https://www.nhs.uk/conditions/diarrhoea-and-vomiting/
This has good advice for what to watch out for.