My take:
If you don't give testimony I see the reverse happening, he will get out while he is still angry, and you won't have gotten very far away, you will be easy to track down and "do harm" to while "the injustice" is still fresh in his mind.
If you give testimony the Police and The Court System will be able to do their job WHICH IS TO PROTECT YOU which you aren't allowing them to do at the moment.
If you give testimony, he will have time to take stock and really think about his actions, specifically which actions of his led him to being there. Also, if over there is anything like over here, there are programs he will automatically be enrolled in if he is convicted of domestic abuse/battery.
If you give testimony, you can get the protection you so desperately need, and quite frankly, he can get the help he so desperately needs.
If he has to spend some time "inside" I promise it will come to him that this is in direct action to his own actions. Take this from someone who seen his fair share of the inside of a jail cell. If I got out within 48 hours or so I was "unjustly held" and went straight to the Pub to knock a few back and regale everyone with a few new stories of the adventure that was my life.
The few times I had to actually spend some "real time" inside, it was impossible to "explain away" as somebody else's fault, and I had a great deal of time to think about what was it IN ME that got me landed in jail.
If you don't testify, you are placing yourself directly in harms way, and truthfully you aren't doing anyone a favor, including him.
There are times when "society" and "the court system" is actually a good and helpful thing, this is a lifelong "outlaw" type telling you this.
I know a number of men who have been "sent up" for spousal battery and the ones that actually do get caught, do the program, do the anger management classes etc. some of them "come out" good men, and actually help others, one just got his degree and now teaches the classes.
The ones that don't continue to batter other women, sometimes with permanent consequences.
Please think about the future, both your own, and the next woman he dates. Think about how you would feel if he harmed her and you had a chance to stop that cycle of abuse.
You aren't "the powerless one" here anymore, you have all of "society" backing you up.
Do the right thing, of course, only you can decide what that ultimately is, but please, for your sake, his sake, and his future GF's sake, please, do the right thing.
We are here for you.