1. Greenish fluid is also Bile,and probably his intestine must be still irritated by the procedure which has resulted in throwing up of the bile through mouth, OR can be due to Infection [if proved otherwise]
2. make sure soft diet
doesn't contains dried beans,onions,cabbage,eggs, fish, asparagus,melons,carbonated drinks and fatty food as they will cause more gas than others.
3. make sure that he chews his food well and doesn't swallow [resulting in Air swallowing] and avoid drinking through a straw, also don't have long periods of fasting as GAS formed is more in empty bowel.