Aww, nine months na pala siya! Honestly, green-yellow watery poop can be tricky kasi maraming reasons, sometimes it's just the formula, sometimes it's a viral thing, or maybe his tummy's adjusting pa rin. I'd really suggest checking with your pedia just to be safe, lalo na if he's acting fussy or nag-lose ng appetite, kasi wala nang guessing game pagdating sa ganito. A few of us mums noticed our babies' poop changed slightly with S26, so it could be that, but better safe than sorry talaga!