Speaking from experience, I was banned for five years for throwing an empty bovril cup over the wall (sitting in the front row) as was a friend. There was no malicious intent but we were made an example of given circumstances around the time. What we did pales in comparison to what has gone on recently at Fir Park and even worse at other grounds.
We weren't 14 at the time and I can only imagine the regret the kid has. We were left with the option of 'supporting Motherwell as we see fit' basically as long as we don't have another brain fart, we'll be fine. The rest of that season I missed one game.
I'm sure the kid won't have a problem getting back in (providing he has friends willing to get him a ticket) and he learns from his mistake.
I'm not condoning throwing stuff onto a pitch, players have the right to not fear for their safety at work but a degree of common sense has to come into it.