D.K.
Babies when teething don't eat as much as normal. I say get some natural teething tablets, they worked great for my son, immediate relief, try giving them to him right before feeding him. With my daughter the tablets did nothing and I had to give her tylenol. Have him chew on a cold/semi frozen wash cloth to provide relief on swollen gums. Teething tablets or tylenol will help quickly. Try pumping and putting in a bottle so you know the exact amount he is taking in.