If he's skinny maybe you can try giving him a little
alfalfa hay (not too much, though). I assume he does eat hay, right?
I would just keep offering different vegetables...eventually I bet he'll try something he'll like

. Some degu favorites seem to be parsley (mine don't really like it, though),
romaine lettuce, carrots, and sweet potatoes (the skin is toxic so be careful). Mine also enjoy brocolli and celery but I don't give those as much.
Oh, and to give you an example on how their tastes can change...my degus never liked cucumbers. In fact, they hated them. But today I tried again and Rosie has decided she loves them now!