Dragon gobies are filter feeders and despite them haveing big mouths they are terrible at finding food as their eyes are so small.
Ours constantly have their mouths open which can be mistaken for gasping but they are filter feeding. Is yours on sand? This is a must with gobies and ours have grown from 3" to 12" in 5 months. We have 8 in a 7ft tank and given plenty of places to hide, ie. rocks they have great fun together.
They need very very finely chopped meaty food ie, prawn, cockle and mussel and like i said, don`t be fooled by the size of their mouths, they can only take small food.
The chopped food should be left in a couple of days as they prefer it after a day or so. Mine do eat bloodworms but don`t seem overly keen.
Hope this helps. What else is in his tank and how big a tank is he in?
Clare