Need help with my Dwarf Gourami

Firstly, is this a new tank? It is not fully cycled if it has nitrites.

Also, the gouramis will often not get along with angels and bettas are a big no no in that tank. Bettas and gouramis, or bettas and angels, are a notoriously bad combination. One day, someone will get hurt. I suggest getting the betta his own little tank.

As to your question, are his scales poofed up like a pinecone? Is his color more silvery than usual? Is it just his stomach or his whole body? Has he been pooping?

If his scales are puffed up its dropsy - usually not treatable and the fish will die in a short amount of time. If its just his stomach and he seems to be active normal other than his lack of appetite, he could be constipated or blocked up somehow. If this is the case, feeding him some boiled skin peas or other greens could help flush the system out a bit. That said, two weeks is an awful long time for this to be the case...

It could be dropsy or plain and simple constipation. Whil in QT, I suggest an epsom salt treatment. 1/8 TEAspoon per five gallons can relieve the bloackage. Try this and watch the fish for signs of a bowel movement. After it has one, offer pre-soaked food. Pre-soaking keeps it from absorbing fluids in the stomach and creating a blockage. While in the Q-tank, see if the scales are protruding from the body, making the fish look sort of pine-cone-ish. If it appears this way and the Epsom salt treatment doesn't seem to have any affect, then it is likely dropsy.