I would make sure he knows you are married, and then stop communicating with him unless it is work / school related, and then only talk about that topic. Guys have to know that a married woman is off limits. Sometimes, tho, they think that maybe the marriage may fail, and so they want to be there to sweep you off your feet when it does fail. If you talk / text him, he may take that as a type of flirting, confirming his distorted fantasy that you may be more into him than your husband. If he continues to to bother you, then get authorities, husband, etc. involved. He is already abusing your "friendship" so don't feel too sorry for this jerk. He may have been a friend at one time, but a friend always wants what makes the other happy; this jerk is selfish, wanting to break up a marriage that makes you happy for his own selfish desires. Anyone that does that to you is no longer a friend in my book, and deserves to be treated like the jerk he is (boy, I really want to use harsher language than "jerk", but it's too colorful for this forum)
As a man, I even get upset with jerks like this when I see them hitting on women they know are married and/or are even just dating someone. There's this one guy that sings karaoke at the same bar that is just a womanizing jerk. He thinks he's God's gift to women, and has even stated that he doesn't care if a woman is "taken" as long as he could beat-up the other guy if it ever came to a fight. Come on! We're not in high school anymore! It seems like the only thing that will get this guy away from a woman is if she becomes extremely cold towards him; if he doesn't feel like the woman likes him, he'll go off in search of "easier prey". I think you need to do the same; show that you have no interest in him whatsoever and maybe that will get him to back off.