I figure a good ten to fifteen years worth of law suits, serious substance abuse, an incredibly fucked up relationship with Sylvia (who is not a mentally healthy woman by any standard) and probably an early death.
He falls for the first cult to offer him peace and stability, gives away all his royalty and settlement cash to them, and winds up starring on Celebrity Rehab.
He also starts hanging out with Andy Kaufman and develops a killer Tony Clifton imitation.
About the lawsuits; the production company has been doing this for 30 years in with a global audience. I think it’s safe to that the legal system has some major differences with the real world. Hell the production company likely had state & federal law changed to suit them; they already managed to get the law changed so that a corporation could adopt an orphan.
Then he’ll just have to get a sheep and a cow and breed horses.
The movie ended because the show ended. We were the voyeurs as much as anyone.
Christof won’t let it go. Eventually, Truman kills him.
Mental breakdown.
Life in an asylum.
He traded one, happy, controlled world, for another, less pleasant one.
One of the other babies, who just missed being Truman because they were not quite born at the right time, tracks him down and kills him.
I like to think his name is Guy Gardner.
Allllll-righty then.
Contracted from Jenny Mc Carthy, natch. 
The movie has so many ridiculous plot holes and completely ignores many legal obstacles that would have prevented the show from ever getting off the ground in the first place.
But, taking the movie at face value, and assuming the studio and Cristof weren’t arrested for their long list of crimes:
Truman would likely find the wide world a larger version of Seahaven in many ways. He’d be stalked and stared at and pointed to wherever he went. I could see a whole industry revolving around filming Truman as he goes about his daily life. If anything, lashing out, drugs, alcohol, etc, would make it more interesting to people.
The actors, extras, and crew would all be out of work. Some would be upset with Truman for ending their jobs, and some would feel guilty for participating in his kidnapping and exploitation.
Cristof, well that’s a tough one. In a universe where you can kidnap someone as a child and then covertly film every moment of that person’s (captive) life while controlling everything they do, well, what consequences could there be for him? He’d owe Truman huge amounts of money, so that could cause some legal battles. I could see him trying to talk to Truman, and Truman getting pissed and punching him out.
I could see Truman going to either extreme. He could move somewhere remote and isolate himself from humanity. He lived a pretty sheltered life, I could see him getting to the point where he was just unable to function in the real world. Or, used to having attention, he could just embrace it, partner up with Cristof or someone else, and rake it in. Truman is shown to be a bit of a ham, even when alone.
He could often be found blatantly looking in windows, following people as they moved from room to room while he snacked on a bag of potato chips, sometimes nachos. Whenever he got noticed he would shout “Hey! How do you like it?” which always made them too embarrassed to press charges.
Realizing how much modern technology was used to screw him, he becomes the wold’s first and only Amish Missionary. It fails because most people don’t hear him knocking and he refuses to use the doorbell. He is later murdered by a Kardashian for stealing the omnipresent audience that should have belonged to them.
This.
It’s not “kidnapping”; Truman was the first child “legally adopted by a corporation”. Mind you, he’d have an interesting case for child abuse - on the one hand, he was well cared for but on the other, he was held captive through psychological manipulation. At the very least he’d have a good civil case for damages (in addition to the back wages he’s owed, of course).
He was last seen on Celebrity Rehab with Dr. Drew Pinsky VIII and VH1’s I love the 2190’s.
The movie actually deals with that (perhaps not realistically, but maybe) - right at the end, the security guards start looking for something else to watch. Truman’s fame has ended. He’s probably a minor celebrity - he’s “hey, aren’t you that guy who used to be in The Truman Show?”
The computer his simulation is running on (The Sims 54) crashes when Windows 17 bluescreens. He is lost for all time since “cloud save” never worked.
The only trace of him left is dozens of YouCube tridi videos.