And hard though it is, you need to encourage him to maintain some type of relationship with his dad. Sometimes they will side with the "injured" spouse, but that can have bad consequences long term (in my ex's family, the boys sided with their dad when their mom left him during their teen years - they cut off all contact with her. Ten years later I encouraged my ex to contact her, since "no contact" was more of a burden than polite contact. One brother waited until his 40's to resume contact, and one in his 50's still doesn't speak to her.)
So better for him to have a polite, if maybe distant, relationship with his dad, than for him to get pulled into "siding" with you.
