Help. My boyfriend wants to "share" me with another man... What should I do?

At your age I would say no, there are too many emotional things that come along with it. Not to mention security, 2 guys and just you, think about it.

I have been with my wife for 4 years, we swing but not often. There is a huge amount of trust with something like this.

If you do decide to do it, here is my suggestion.
You pick the guy
You pick the time
You pick the place

Your boyfriend is to approve of the guy.
The 3 of you sit down at the coffee shop, in a quiet corner and you explain exactly how things are going to go down, and stick to your plan the first few times.
Set your rules in stone! no anal, no kissing, no whatever your rules are. If you say no at any time, all parties must stop what they are doing, no matter how far you have gone.
Figure out the issues and move forward.

Dont forget there will be drama, oh yes there will be. The jealousy monster is a green eyed one horned beast, and he will rear his ugly mug, you can count on it.
Like I said, there is a lot of emotional stuff here, and I dont think your mature enough to handle it. No offense meant at all. Just speaking my mind.
Good luck!