I'm with Majmike on this one.
If he gets away with a soft punishment it won't mean shit to him, and he'll carry on 'til the next load of shit hits the fan, do you want to be there when it does? It's about maturity, it's about realising that there are things you can do and things you don't do, and one day he'll cotton on, but he might be in his 30's before he does. Do you want to hang around outside the gates 'til then? Life's too short for that, yeh I know that at 17:-
You are young and life is long and there is time to kill today
But the song goes on:-
And then one day you find ten years have got behind you
No one told you when to run, you missed the starting gun (Pink Floyd)
I had a totally clean record at 17, then a series of minor offences (Obstructing a police officer, petty theft, drug possession) led to me ending up spending over 60% of the next 10 years in prison. If he isn't going to change NOW then move on, your number 1 duty is to yourself.
Hope I haven't sounded like yer grandpappy.