Rant away. It's very good for your ds to have a relationship with his paternal grandparents. Keep doing that despite your ex rather than because of it.
I can't offer any positive advice. I think you just have to accept he's useless and don't rely or hope that he'll change. People by and large don't change, certainly not overnight. I think it's easy to be mislead and to not really know someone though and to be really surprised when you find out what they're real strength of character is like.
My ex was shocked when I fell pregnant but was adamant he'd stand by us and do the right thing. But no, he stopped contacting me before my ds was even born. I had to threaten him with court to get any financial help after I felt I had waited long enough for him to volunteer.
I sent him photos, father's day cards etc, but I don't any more. I asked him about a year ago to reconsider his choice to have no contact as my son was asking a lot of questions and wanting to be in touch with him. He refused.
His parents have been great though. Sending clothes, toys, books etc. They never forget a birthday, christmas, easter and several other special events. When my son asks about his dad I explain he has a father not a dad and always finish up by saying that he has a great family who love him lots, me, his grandma and grandpa etc...
It was hard for me to let go. I think because I knew it would be upsetting for my son to hear the truth. But in reality it was harder for me to tell him, he took it in his stride. I'm sure I'll have to answer more questions as he gets older. But I've accepted now his dad won't ever be part of his life. If he appears at some point that'll be great, but I'm no longer waiting or hoping for it.
Gilly