Don't worry about solids for a good 72 hours after the puking stops. If a child is throwing up for over 16 hours my Pediatrician wants to see them. Clear liquids only, nothing else, water, gatorade, pedialyte, clear low sodium soup broth.
Let him go five hours without throwing up before you offer any solids, then only dry toast, crackers or something very mild.
I have endured my share of tummy viruses, not fun however I have gotten it down on when to be concerned and when to ride it out. If your son has a fever, is throwing up for more then a whole day, call your Pediatrician. Talk to them about what is going on and let them help you feel better if he needs to come in or not.
Don't give him milk at all, no dairy until his tummy settles, clear liquids in little sips. Every time he pukes have him sip out of a straw to help keep him hydrated.
My son went without eating solids last spring for five days, lost like 5 lbs due to a bad stomach flu, however main thing was to ensure he was hydrated and when he got better he totally made up for the weight loss! :) Hang in there, hugs!