I was in the same boat bro - I had a bad case of the mumps a while back and lost a 17 pounds in 6 days due to a stupendously highly active metabolism, and its still the same today I'm still only 11 stone but I went right down to under 9.5 in the space of a week.
What I eat everyday now is
breakky - porridge/museli/nuts and seeds collection
midmornin - apple/banana
lunch - rice/rawveggie
midafternoon - (blue, black, rasp)berries/orange/necterine/plum etc
dinns - mostly fish but sometimes chicken too / brocholli, cauliflower, peas, carrots etc
beforebed - some nuts and seeds
and drink at least 2 litres of water throughout the day, have between 2-4 cups of lemon balm tea, and likewise for green tea, around 6-8 cups of tea a day - and treat myself to some apple juice or pomegranate juice too!
Do this and you should gain wait! but eat PLENTY OF GOOD STUFF! Good luck!