No; you cannot be forced to testify, but keep in mind that you and your friend ARE the prosecution's entire case and without your cooperation to testify on the stand the case will simply be dismissed against the accused.
More serious than perjury, is the crime of threatening and intimidating witnesses in a criminal case which is what the defendant is committing by calling and harassing you and at this stage of the game, it is very important that this act be reported to the police as well. He has already tried to put you in jail by filing a false report of basically attempted murder and he is now harassing you over the phone and I am sure he does not call to talk about the wheather or invite you over for Thanksgiving dinner. So, if this individual is as mentally unstable and devious as he sounds, then what stops him from showing up at your door to confront you personally or pull some other irreversible acts of criminal stupidity?
Normally, telephone conversations cannot be recorded save a court order tap, or if recorded will not admissible in any court proceedings. But in this situation when the individual is already being prosecuted for a criminal act who is also threatening the witness to boot, not only are you privileged to record his calls, but it will also be admitted as evidence to prove further culpability by the defendant.
Your nervousness for going to court for the first time should not be a factor in your decision as to be a witness or not, but whether you would be opening the door to more harassment and violence from this guy by refusing to testify. Not to mention the old age cliché about the kind of message your withdrawal as a witness will send to the this person in light of his harassment.
But at the very least, take precaution and do yourself a huge favor and go take out a restraining order against this person.
fredrikklaw