Encourage him to eat it as much as you can. Maybe add a little warm water to soften it and/or top it with some Chicken with Chicken Gravy baby food.
If he won't eat it, you can feed him some Little Cesars (wet in the little packet at Wal-Mart and grocery stores) or chicken or chicken with rice. The yorkie I have now got really, really sick when she was teething. Worse than I've ever seen in any of my doggies. She even threw up a few times. I can even remember a couple of days where she just sorta slept most of the day and looked pitiful, but I knew it was her teeth. My vet said some just have a harder time, just as some people do.
It took awhile, but I finally got mine back to her normal eating.