Why do you feel he is skinny? Has he lost weight, does his spine and hip bones feel sharp and not rounded?
How old is your rabbit and how much does your rabbit weigh now and how much did he weigh when a healthy weight? What amount of pellets(1/8, 1/4, 1/2 cup, etc) and what type(alfalfa or timothy based) does he get each day and does he eat it all? Does he get unlimited amounts of hay and can you verify he is actually eating it and what amount(pile the size of his body per day, half a pile, etc)? Is his hay mostly leafy, mostly stalky, a mix, and what type is it? Any other treats, veggies, foods? Have you changed anything in his diet, different kind of hay, different kind of pellets? How long since you noticed the weight loss? Is he showing any difficulty chewing or other
signs of dental issues, or
other health problems like increased thirst/urination, changes of behavior, signs of pain, etc?
There are several things that can cause weight loss in rabbits, and if diet is ruled out, everything else is health related and would take a vet to investigate.
http://www.rabbitnetwork.org/articles/weightloss.shtml