im in about the same boat as you, i eat all the time, always hungry, and rarely do any exercise and I cant gain weight.
i have been the same weight since I was about 14. last year i lost 7kgs, i managed to get that back but am unable to get any higher. I AM under weight but I cant fix it.
when I put the 7kgs back on i was drinking sustagen hospital grade shakes. But they stopped working once i got back to 'normal'.

If you find anything that works, i would love to try it. Though I know mine is a high metabolism thing, but i cant see why that stops me getting more than i am. and i havent lost weight since then either :/