How much of what I told police can they use against him?

All of it. Just so you know, prosecutors will never drop charges just because the victim has had a change of heart/story. The fact is, in 95% of all DV cases the parties reconcile, and prosecutors are well aware of this. He needs a good lawyer.

How much of what you told police can they use against him? Everything. With regards to the police not reading him his rights, although an officer should read you your Miranda rights when you are arrested, it is not absolutely necessary as long as you are not questioned about the crime for which you were arrested and/or were detained during the questioning. Miranda only acts to suppress any statements you gave the police after you are arrested or detained. It does not invalidate an arrest.