S.H.
Teething "pain" begins even before you actually see the teeth erupting.
When teething, babies often seem to, "reject" nursing.
Both my kids did that, when teething.
But I still nursed on demand.
And made sure they did not get dehydrated.
Don't overdo the Tylenol, unless you know the exact dosage and frequency, for a baby that age.
Your baby also knaws on the sippy cup.
Try giving him a teething toy.
Or, my kids liked to chew on a frozen little washcloth... that I would dampen and put in the freezer.
Teething is temporary.
Or, he might be ill.
Does he have an ear ache? This can also affect nursing.