Generally speaking, it is not against the law for a person to talk with prospective witnesses to an incident. If there is tampering with a witness, or coercion of the witness, that would be a different story. You should consult a criminal defense attorney in your jurisdiction to make sure you are fully aware of your rights and obligations, and to have a professional at your side to speak on your behalf.
Given the facts that you relay in your post, it is permissable for the ex-boyfriend to call witnesses to ask questions. The witness can refuse to answer his questions, however. It is entirely up to them. You have the same opportunity to call his witnesses.
*** This is a general interest posting on a public website, regarding general legal concepts. This posting is not, and should not be interpreted to be legal advice to anyone, or to create an attorney client relationship between the poster and anyone. There is no way the amount of information in a website posting could be sufficient for the poster to provide informed legal advice. If you have legal questions pertaining to your situation, you should discuss them with an attorney, in person, and in sufficient detail for that attorney to provide you with confidential, competent legal advice ***