It is not possible to hazard a guess at what sentence he may receive as you have not said what offence he has been charged with. However, that is not the most important issue here.
Mr Veritas and northernmonkey are quite correct. Statistics show, generally, that perpetrators of domestic violence do not change for the better. More usually if left unchecked their behaviour becomes more extreme, more abusive and more violent. The victims always believe they will change. Rarely are they right. If you are not prepared to ditch this “good bloke” it is very likely that you will be on the receiving end of his increasingly violent behaviour.
The police are not there to deal with you just because you get upset. If your gentleman friend has committed an offence against you they will prosecute and, if sufficient evidence is available, they will do this with or without your co-operation. You may find yourself subject to a witness summons to go to court to give evidence.
Before you worry about what sentence your good bloke may receive you really need to think about how you’d like your future to pan out.