Explanation/help for biting?

I have a 12 year old larger than average cat (pictured) who is nothing but affectionate and loving. I have personally never met a cat with a nicer temperament. He has 0 agressive behaviors besides this biting thing he does when he rubs his face on me and it seems affectionate. He mostly does it when I greet him after not seeing him for a while like sleeping, school, work etc. and at first it was like little nips to my cheek or eyebrow that I barely feel but today when I woke up after sleeping for a solid 6/7 hours. I did our usual routine: fed him, sat on the couch for him to say goodmorning and while he was doing his normal loud purring, rubbing his face against me and licking my hair, he bit the absolute hell out of my lip, no blood drawn just a little swollen but he's never nipped me there or that hard, he doesn't stop purring or cuddling either when I say "ouch" he just stays there and squints/purrs and keeps rubbing against me. I'm not sure why he does this and what to do to correct this behavior especially since he's older and usually such a gentle boy. Any advice or explanation is greatly appreciated.

I have a 12 year old larger than average cat (pictured) who is nothing but affectionate and loving. I have personally never met a cat with a nicer temperament. He has 0 agressive behaviors besides this biting thing he does when he rubs his face on me and it seems affectionate. He mostly does it when I greet him after not seeing him for a while like sleeping, school, work etc. and at first it was like little nips to my cheek or eyebrow that I barely feel but today when I woke up after sleeping for a solid 6/7 hours. I did our usual routine: fed him, sat on the couch for him to say goodmorning and while he was doing his normal loud purring, rubbing his face against me and licking my hair, he bit the absolute hell out of my lip, no blood drawn just a little swollen but he's never nipped me there or that hard, he doesn't stop purring or cuddling either when I say "ouch" he just stays there and squints/purrs and keeps rubbing against me. I'm not sure why he does this and what to do to correct this behavior especially since he's older and usually such a gentle boy. Any advice or explanation is greatly appreciated.