M.C.
If the teeth are in, he may have others coming too. Try giving him Hyland's Homeopathic Teething Tablets. My son would stop nursing and eating every time he was teething (whether or not it was evident to me). I would give him the tablets and his appetite returned. (I nursed for 2 years).
Also, consider...have you increased his solid food intake? If so, expect him to nurse less frequently. With my first child, I did not consider this and thought she was weaning herself. Unfortunately, nursing ending by age 1 unnecessarily!
Mom of 4: girl, boy, boy, baby due 10/1