First degree domestic assault is a class b felony, unless the victim is seriously injured, in which case it is a class a felony (RSMo 565.050). A class b felony has a max sentence of 5-15 years.
Second degree domestic assault is a class c felony (RSMo 565.060). A class c felony has a max sentence of up to 7 years, fine of $5,000.
A conviction for ACA carries a minimum sentence of three years for a first offense (RSMo 571.015).
All that being said, the situation and the specific facts can of course mitigate the length of the sentence and possibly even the charges themselves. No one here will be able to tell you what kind of time he will be facing, only provide you with the generic info above. The prosecutor, the courts, the facts - everything can influence how much time your son will actuall face.
Your son needs to get an experienced criminal defense attorney ASAP.