Magistrates’ sentencing guidelines for this offence as you describe it suggest that their “starting point” when considering sentence is a fine of one week’s net income. However, this is for a first time offender pleading not guilty and having been found guilty at trial.
If your son pleads guilty at the earliest opportunity he will receive a discount of one third off the fine. He will also pay a £15 Victim Surcharge, around £45 costs and he may be ordered to pay compensation to his victim for the damage caused.
Having said all that, bearing in mind this is your son’s first offence, the Bench may feel able to deal with the matter by way of a Conditional Discharge. This means that there will be no immediate punishment for this offence (although he will still be liable to pay costs and compensation). However, should he reoffend within the period of the discharge (which can be any period from 6 months to 3 years, with 12 months being the most usual) he will be punished for the new offence and for this one as well.
He would do well not to make too much play on the fact that he was drunk. Courts consider this to be an aggravating feature and it goes no way towards excusing the behaviour of those accused.