He should be back at the Bard's College. He's always in there when I return from the reading of the poem at the Blue Palace. If he's not there and you're on PC you can try either of these two commands:
prid 000198CFmoveto playerwhich should move him to your location, or
player.moveto 000198CFwhich should move you to his location.
Have there been any dragon attacks on Solitude, or did you finish the CW on the side of the Stormcloaks? I don't know if Jorn is essential and I've never heard of him being killed off by anything that happens in the city, but if he's completely missing from your game that would be the only reason I could think of for it. Unless he *is* essential in which case he should be at the Bard's College. Does the marker make it look like he's in the College, and if not, where is it pointing?