Teething definitely affects breastfeeding. Some babies will refuse/go on a nursing strike or will do what your lo is doing now while some want to stay on breast for hours.
How long does he nurse for? Is it less than 10 minutes and then he is bobbing on and off? I would say that short feeds are typical for this age.
Bobbing on and off would also be possibe because of too fast or too slow milk flow. When the milk flow is too fast they try to escape the jet that goes directly into their throat. When it is too slow, they get impatient.
Well, you know him better, I would probably vote for teething. As for Tylenol, I am sorry, I have never used it for my dd, so no experience with dosages and age requirements here, I suggest offering teethers that are filled with some gel and can be put in a freezer. Chewing on a cold teether can relieve some of the pain, too.
Good luck!