If he will take the breast when you're together, then give him that unless you have a medical reason not to. I understand that with working that you many not be able to do it full time. Breastfeeding is not an all or nothing proposition.
For teething, I would recommend Hyland's teething tablets. Very little mess and no stink like orajel. Also, you can take a clean washcloth, knot the ends and dip the knots in either expressed breast milk or formula (if you're using that) and freeze. That way he's getting teething relief and some sort of nutrition as solids at that age are nothing but bulk and empty calories.
You can also try transitioning to a sippy cup. The soft spout Nubby cups are great as are straw cups since breastfed babies already know how to suck well.