It definitely could be reflux or likewise protein intolerance is a possibility. Reflux peaks around the 3-4 month period - has it been getting worse? He could be a happy spitter (ie what he's bringing up isn't acidic and causing him pain) In terms of it being an intolerance, what type of formula is he on? If it's cow's milk based, try switching to a soy-based one for a week and see if there's any difference. The only challenge is that around half (I think) of children who are intolerant to cow's milk are also soy intolerant. If the fussiness is exclusive to his feeds, and you've tried soy-based milk with no success, the next option is to trial one of the hypoallergenic formulas like Neocate. That's what I've had to use with both of my girls as they were intolerant to my breastmilk even when I followed a strict exclusion diet!