My son has a huge blister inside his lip?

My son has a huge blister on the inside of his lip,it looks like a canker but his lip is swollen.Looks like he has a fat lip.He has had other ones before but never big,always small.He has allergies and his nose is plugged so he sleeps with his mouth open.Could that be the cause? I can not take him to the clinic or the doctor they are closed.Should I bring him to Emergency room or should I wait till the clinic is open tomorrow? Has anyone ever experienced this before? Thanks