It sounds like you and your husband are doing all the right things - and it's good that you're so supportive of him, as a father. It's not an easy road, but especially if that little boy is hearing bad things about a good Daddy (starting at such a tender, impressionable age), it is absolutely essential that he know, as he grows up, that nothing his mother can do will make his father give up and walk away from him. All kids need to know they're that important to their fathers, but it takes a lot of work to show that to kids whose moms are trying to sabotage the paternal relationship.
With any luck, your Law Guardian will help nip the problem in the bud with adequate visitation. When my step-son was 5, the Custodial Evaluator advised the Court to give my husband more visitation than what's recommended in the state guidelines, "to offset (the Mom)'s efforts to damage the father-son relationship". Then she moved across the country when my step-son was 6 and was quite open that she didn't like my husband getting so much visitation and the main reason she was moving was to get my step-son away from him. The C/E recommended my step-son remain here with my husband, but the court let him move with her anyway. She was so resistant, uncooperative and sometimes flat-out defiant of the court, regarding my husband's visits that my husband won sole custody when my step-son was 8. The C/E stressed to the court that the only reason my step-son had maintained any decent relationship with my husband at all, from 6 to 8 - and a large part of the reason my step-son seems so well-adjusted, considering all the crap he's been exposed to - was my husband's strenuous efforts to maintain contact and regular visitation, no matter what his ex threw at him, to block it. Regular contact is so important.
As to why the ex is doing this... ???? I believe my husband's ex has a profound, core insecurity. She had very upsetting, fluctuating relationships with everyone in her childhood family. They go a decade at a time without speaking to each other. After leaving them, she flitted from one would-be benefactor to another - relatives, boyfriends, fiances, husband, never on her own - right into her 30's. I think she was looking for that secure, supportive connection she didn't feel from her parents, but sadly none of those subsequent relationships worked out, either. I think the single relationship in her entire life that feels unbreakable to her is the one with her son. She appears, in what she says and does, to be incapable of separating the two of them in her mind and to assume he thinks and feels everything she does. So it's easy to imagine that, a) she doesn't understand or accept him loving his Dad, if she hates him; and b) she feels overwhelmingly threatened by my husband spending time with him or being close to him. There's no reasoning with her, because it's not logical. Finally, the only thing the court could do was take control of the situation away from her, by giving my husband custody...which of course confirmed her every worst fear - that my husband always wanted to take her baby away from her. (He didn't. It broke his heart to live separately from his kid, but he appreciates the mother-child bond and he would have accepted regular visitation, if she had simply allowed him to exercise it!) Even worse, by the time he got custody he'd met and married me, so I just add to the threat, by being a potential replacement for her. Maybe similar forces are at work in your situation?
Cameron's mother has always bounced from her parents house to her boyfriends' houses and then back again. We think where she is at now is her brothers house, but we aren't sure. So the insecurity factor may well be what is behind all of this . . . Something to think about anyway . . .
Thank you for your insight.