There's no way to determine how much time he will do, if any without knowing much more information. How bad was the injury? Was there an injury? Was it treated by a medical person?
The best person to ask is an attorney who is representing him.
Even if your Mom drops the charges, the State now has jurisdiction and can still go forward, although they don't always do it.
Good luck and consider someone else who doesn't rely upon his fists to settle arguments. It only gets worse with the passage of time.
The key to potential active time is knowing how much of a sentence he received on the underlying offense for which he is on probation. If the judge ends up "activating" that sentence because the assault on you is a probation violation, then the judge could give him the entire suspended sentence. I think it would be good if you were able to get the assault charge dropped or PJC'ed, and then it would probably not be a probation violation for him and risk activating any sentence. The assault charge itself is not very likely to result in any active time as long as you appear in court too and ask the judge to be lenient and the DA just dismiss it.
The misdemeanor conviction for assault and the probation violation are two separate and distinct sentences, although the conviction of assault may be what “activates” his suspended sentence. The probation violation will depend on the sentence that was suspended when he was placed on probation. The current sentence for assault will depend on several factors, including your age, his prior number of criminal convictions, the severity of the assault, and the District Attorney.
The preceding is offered as general legal advice based on the information provided. As such, any reader should contact a licensed attorney to discuss the specifics of their case and how it relates to the law.