My son was FTT from about 5mths-as in losing weight every week, going from 50th percentile to off the bottom of the chart. He was born normal size, on the 50th percentile. He was happy, healthy and achieving his milestones well ahead of "schedule". He had lots of testing and it was concluded that his body simply burned more calories than we could get into him. He was VERY active. We saw Dr's, MCHNs and dieticians and had a stay in the childrens hospital. In the end we started seeing gains after we increase the calories in his diet/milk. We added Polyjoule (available on prescrition from the chemist) to his formula, and also to his food. We also calorie-added to his solids by adding fats, such as avo, cheese, butter, cream etc. It felt bizarre adding all those things! But it worked, and as he got older we found he was able to eat more volume to get enough calories. At about 4yrs he finally crept onto the 3rd percentile line on the chart, and now at 6 he is back to the 50th percentil and doesn't stick out as the tiny kids in the class. He eats like a horse still, and remains extremely active.
I would def. recommend seeingthe Paed and dietician to get further advice, as the prob is quite common and I know how worrying it is when your LO keeps losing weight when they have none to lose