Star Trek

From what I remember about the Cardassian culture family is very important to them. I think Dukat even brought this out and we saw his affection for his daughter. So it's surprising that Tain had no love for his son. Was it ever explained why? Where did his disdain for his son come from?

There are two things here:

1)  Tain was the head of the Obsidian Order.  He didn't want family to be used against him.  Tain couldn't afford to let familial ties/emotions to kick him out of power.

2)  Garak defied Tain (Although that was secondary as Tain wouldn't even admit Garak was his son before that.)

Agreed. Even as Tain was dying he couldn't imite Elim was his son.

he never called Garak his son but at the end he did recognize him as his son

Another thing to consider: Family is very important to Cardassians, but you can't assume by extension that all Cardassians feel the same way about it. Some Cardassian men, such as Ghemor and Pa'dar, care very much for their children. However, someone like Tain is probably the personality type who does not or even cannot get close to anybody. Interesting (and telling, I think) that Garak and Tain have different surnames. I wonder how that came about.

[Yes -- I have read A Stitch In Time.]

The most likely explanation would be that Tain was not married to Garak's mother.  That would also partially explain Tain's reluctance to acknowledge Garak as his son.  After all, look what happened to Dukat when he acknowledged Ziyal.

(I have not read A Stitch in Time yet.)

Garak was born out of wedlock if i remember correctly. Which is why Tain never fully acknowledged him as family.

Was a great/interesting relationship. One filled with both pride and love. The Dominion war almost brought them closer together.