Thank you for your query.
You can continue taking the medicine and it shall have not effect on your breast feeding son.
It is very difficult to tell the reason for the blisters on your son's lips without having a look at them. They could be oral thrush
and it would be advisable if a health care professional has a look at your son to confirm it.
I hope I have answered your query to your satisfaction.
Wishing you all the best.
Rajiv K Khandelwal