I hope your husband is keeping a record of these discussions with his ex, and the number of times he has tried to see his child. Take the diary and the divorce judgment to a local custody attorney to discuss his options. If you can prove that the ex has been alienating the child against your husband, there is a possibility of getting custody. Good luck.
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It's going to take a while for things to start getting better, even with parenting time enforced.
The way I see this proceeding is that you would file (preferably through an attorney, who can navigate the system for you) a petition to enforce parenting time and get a hearing set. Your husband's ex will then likely file a response to that petition in which she asks to modify the court-ordered parenting time based on any of various things she decided to assert. A hearing would then ensue (assuming the required attempt at mediation fails, and following perhaps a parenting time study) in which the court would determine the best interests of the child. The ex will probably fight hard to keep parenting time to an all-time minimum, stressing the child's desires.
The good news is that parenting time will quite likely end up being more than the every two months the ex is allowing, and the court will not look kindly on her behavior in all this. The child is probably shy and withdrawn because a relationship between him and your husband has not really been allowed to develop.
The way your husband's ex has been behaving, you need to find an attorney to fight for you on this. The ex would probably get an attorney, as well, and you don't want to be stranded, so to speak. You should consult with someone right away.