My first thought would be some sort of metabolic disorder. He is using all that energy for something other than growing. Which could be cause for concern.
As for shrinking I had that happen to one of my DCG a few years back at her 15month appointment she was a bit shorter. Her parents came back from the appointment seriously concerned. Until I pointed out that measuring toddlers is a nurse lying them flat and marking their head and feet on the paper bed liner and measuring it. Major room for error. A little shorter likely just means they over measured at 12months and under measured at 15months. The same may be happening here.
If this were my child I would definitely be following up with more doctors to rule out any disorders. It may very well be nothing but no point waiting a few more years to realize there was something wrong that could have been dealt with. It is great that he appears normal on all other fronts and is happy and well adjusted. But, if he is eating a lot of food, always seems hungry and isn't growing up or out then I do think something could be slightly amiss!!