If he didn't reply he's just arrogant/ignorant!
Even if a non muslim says salam to me I always reply, some may disagree, but my reasons:
1.It might put them off saying it again [as it is a beautiful custom, anyone should say it]
2.It might make them feel embarrassed when they get blanked/ignored.
3.This is quite a wishful thinking point, but still, it may inspire them about Islam, and after seeing the good conduct, he may embrace Islam [Insha'Allah one day]
Now, the problem here is recognition. I always wear atleast something that shows that I am a muslim...Either the kuffi [Hat] or the full armour [

u kno what i mean

].....Soon a beard [which seems to be growing at a thundering rate now :happy:]....
But spread salaam to everybody...If they don't reply, say to them:
"It is waajib for a muslim to reply to this greeting"
He/she should get the message after that