Well personally I wouldn't chastise him for it. He is paying for his son and is a good father to your daughters, he is technically doing what is required of him but evidently doesn't feel a bond.
It's hard to tell without having a greater insight into his personality, i can see from the comments that most will just assume he is a bad person who mistreated his ex and doesn't care about his kids, but assuming you consider him to otherwise be of sound character I'll play devil's advocate and suggest why he could be acting that way.
Everyone likes to say that being separated from the other parent and only seeing your child for a handful of days in a month shouldn't effect the bond you have with them, but that isn't always the reality. For some, it is devastating - being a father was very important to my partner but he has never been able to have a particularly positive bond with his son despite trying for years, a lot of which has to do with not having much influence over the kind of person he's becoming and the person with that influence being his ex who he hates. He didn't get on with his mum so the more similar his attitude is to his mums, it's only natural that they would also not naturally get on, so the relationship is strained.
Your partner didn't see much of his son in the early years of his life and let's be honest, that could very easily effect your bond with a child. A lot of import is put onto dads having the opportunity to bond with the child they themselves didn't carry early on. Not only will he not have a strong, established connection with him from the early days of caring for a newborn, but he will most likely have had to actively move on from missing him, and let go of the idea of being close to him, during that time. In the early days, my partner spent a lot of time being very, very sad over missing his son, which is perhaps something your partner is trying to protect himself from, especially since he has lost all contact before so knows it can happen.
By all accounts, being very close to a child you rarely see who you struggle to get along with because of issues with the mother is a recipe for heartache. So I would be gentle with your partner about his decision to avoid that.
He needs to ensure when he has his son he is treated warmly and also not foisted off onto other people, however, as that isn't fair on them.