I feel for you.
I'm a friend of an ABDL, but that is only recent, which is why I am on here. Your marriage and loss of trust speaks more to me because I am recently divorced (2 years ago, after a 14 year marriage) and went through years of my husband lying to me, especially about porn, even about an affair.
You've got a couple different things that might be going on. One, your husband has probably had a thing for diapers for years, and may have never really experimented with them. And now that he sees them and is using them for the baby, he may be more interested in experimenting with them.
Having a kid takes time away from your relationship too, so he may be spending more time on the computer as a passive aggressive way to get back at you for spending so much time with the baby. Maybe jealous to a certain extent?
I wouldn't worry too much about him being sexually interested in the baby. Especially if he has owned up to an interest in diapers already (prior to the baby?). He may be thinking more about his own childhood and regressing some himself.
Have you played with diapers before with him? How comfortable are you with his fettish? You might be able to engage him with his fetish and provide him an outlet other then the computer.
Your welcome to PM me if you want.