very concerned about his lack of weight gain

My 4 year old is underweight for his age (height is ok) he has loose very smelly and very dark stools. Sometimes has has very smelly farts and burps a lot. We are very concerned about his lack of weight gain. We have been giving him an iron and vitamin B tonic, this seems to make his stools better. We have had his stools and blood tested twice for coeliacs, first test was positive second test was negative (12 months between tests) 2 Years ago we took him off wheat and this also improved his stools, but we have just recently had him tested for allergy to wheat and lactose, this also came back negative. Please help us with this as I say we are very concerned that there is something wrong with him that is stopping him from developing correctly.