The sentence, if the facts lead to a conviction, will be based to a large degree on his history. A Malicious Mischief in the Third Degree is a gross misdemeanor, meaning the Court can sentence him anywhere up to 1 year in jail and up to a $5,000 fine. It is hard to predict how a judge would come down without knowing more facts.
His best course of action would be (a) not talk to you because I'm sure there is a no contact order, (b) get back into treatment, and let the provider know of the relapse, and (3) get in touch with a good local attorney who can explain to the Prosecutor his struggles with addiction and paint him as a person rather than a high-file number. Hopefully all works out for your family in the long run.
Your son's father needs counsel with experience representing people charged with domestic violence offenses. While the maximum penalty is one year in jail, the ultimate sentence should take into account both his criminal history and any mitigating circumstances, such as mental health issues. Both he and you might also benefit from you retaining counsel of your own to help you voice to the prosecutor and the judge your wishes regarding a resolution of the case.