i had a problem with a guy like this once, ended up at a house party one night and somehow ended up sitting next to him. Turned around and asked him why he had a problem with me. "dunno" he said, haven't seen him since.
I understand not all situations end up like that, don't bother sending him mesages or calling him, people like that only understand size and power. Get yourself some weights. Atleast that way you atleast look like a threat. I'm 6ft2 and i weigh 160lbs but the difference is i have no fat on me, just muscle. I've never been hit twice because i can take a punch and look straight back at them.
Most fights however i can talk my way out of. One time this guy started on me because i was carrying his missus around the club on my arm talking to her, i put her down and he comes over, starts making threats. I start texting her making threats to him. When i walk out of the club, he's there waiting for me. He tries to start a fight and i ignore him and start chatting to his mate, who completely saw my side of the argument and sided with me that he was just being jealous (i knew the girl by the way). Anyway the guy goes for me, pushes me backwards before being knocked over by my mate (a bouncer) he slumps off while i turn back and carry on talking to his mate. Anyway, somehow he'd scratched me and i don't like wearing bloody shirts, that's not cool, so i ended up walking around town at about 3am with no shirt on. I saw him and his missus later on (outside a kebab shop) so i went up and started chatting to her, completely ignoring the fact that he was there with a group of mates standing behind him. He didn't dare say a word.
Point is, don't show fear, act like he don't mean shit to you. Sometimes playing the hard guy can be the best option, unless of course it back fires. and having the right group of friends helps to!