You simply tell him you are NOT interested.

Simple as that. You don't need to take it any further by explaining why. But if you feel that you need to, then tell him:
1. He's married
2. He's been a friend to you. Nothing more.
3. Your personal matters are your own business which you share with no one.

Dang. Some people never cease to amaze me.

Good luck.

Shangrala