It’ll most likely be the snotty nose as he’ll be struggling to breathe when feeding. We’re in the exact same boat atm, I’m using Calpol nasal saline spray to clear his nose before a feed but I just keep offering boob. If he goes a long time without a feed, call 111 and get some advice. The last time he had a cold, we ended up down the hospital and were facing tube feeding as he was struggling so much.
Yess could be teething and along with the snotty nose might be hard for him to nurse properly. Maybe give him some calpol / teething gel or powder and try to clear his snotty nose and then try to nurse him, it’ll pass and he’ll be back to normal x