Without knowing exactly what the underlying cause is, I cannot provide specifics. Here are some general recommendations you may wish to try:
When vomiting or diarrhea, it is important to replenish electrolytes.
Check out this recipes here for that: http://www.nutriliving.com/recipes/hydrate-orade
If it's bacterial, some good ingredients to include that are anti-microbial are:
cold-pressed virgin coconut oil, turmeric, ginger (helps with stomach upset and nausea), and oil of oregano.
Upset stomach often times leaves one with a low appetite - try the BRATT diet (bland foods) consisting of bananas, rice, apple sauce, toast and tea - these are usually well tolerated to get some basic nutrients into the body.
Maybe try a blast with banana, 1/2 an apple, a touch of ginger, some oats, and coconut water. I haven't tried this one, but doesn't sound too bad ;) I'd skip the leafy greens for now as these are difficult for some to digest when the tummy is not feeling well.
We hope he feels better soon!