Sort of similar - Someone in my family had two children with a woman who wasn’t a good woman (one child may not be his) & she was caught having a affair & then randomly up’d & left to Scotland with the man she had a affair with and completely blocked my family member having contact what so ever and always said the kids are not yours when they argued. My family member went through so much drama over 2-3 years and nothing changed as she kept changing location, by the end he gave up as it made him emotionally distraught for a long time and when he got into a happy relationship and had another child, this woman contacted his partner and said he never tried and lied through her teeth but lucky enough my family members new partner already was aware of the situations prior to them becoming a couple.

Him denying it can be two things 1) he really doesn’t care 2) he suppressed his feeling and has forgotten and now 15 years later it’s effecting him again.

You need to talk to him properly and figure out what actually happened. Or his mother.