Wouldn't recommend "crap". Depending on the usage, he could say "He doesn't know jack", or "Dang it" instead.
As for "damned", again, you can replace it with "dang it". But if it's used like "that damned thing", I think it's okay.
For "hell", just use "heck". It's not that bad.
Remember, think context. What is the character trying to say? Is he angry, insulted, excited? It'll give you a better clue.
Also, 14 years old sounds a bit old for MG readers, regardless of who it's aimed at. 13 might be more acceptable. Is there any reason your MC can't be younger or your target audience (voice, plot, etc) can't be older? You can get away with those words in YA.
P.S. I just revised a scene on my MG fantasy where the character is trying to repeat something someone told him, but he vaguely remembers so he says: "Lost my marbles. Can't talk. Have a plan. Balls."
I expect the "balls" part might be misinterpreted (the character he's telling this to is taken aback, lol", but then he corrects himself and all is well.
Don't plan to change it.