My baby has acid reflux and has not been feeding well- he either cries at the breast or falls asleep. He is mostly feeding for less than 5min and feeding infrequently. As such, he end up drinking only the foremilk and I noticed his poo is now green. FTM, so maybe my question may sound silly, but can I express out the foremilk until the milk is noticeable less watery/more 'white' before letting him latch to increase the chances of him drinking the hindmilk?

Acid reflux can be due to sensitivity to your diet, and so is green poo. Foremilk-hindmilk imbalance, resulting in green poo, is not as common as everyone thinks, unless you are a severe oversupplier. Another possibility is that you have an overactive letdown, resulting in your baby latching for less than 5 minutes. In that case, it would be advisable to pump for a letdown first and then latch your baby. Or, latch your bub lying backwards so that the gravity can help with the overactive letdown. Hope it helps.