Just rip the bandaid off.
"Sorry man, I've been pretty busy these past few years. Can you tell me your name again?"
Then it goes 2 ways:
-
This guy is a friend of a friend you have only met a couple times
-
this is someone you used to hang out with a lot and drifted apart from, and they changed a lot over the years.
Then, if it's option 1, make small talk with basically a stranger. If it's option 2, then you get to pull the "Wow! I didn't even recognize you, man!" line.