I think it's unlikely to be lactose intolerance. If babies have that from birth, they cannot gain weight at all without medical intervention. It's often misdiagnosed when there's actualy a probably with milk proteins. Otherwise children can develop lactose intolerance later on - after the age they would have weaned (school-age-ish I think).
It does sound like he has more than one issue, though. What a pain being so far from medical experts! I think you need an expert peaed/dietician to help identify the problem.
There could well be fructose in the formula too...