What did making the son disabled add to this?

Big picture? Not a lot. But the pilot episode uses it when the bullies are picking on him. They could have given the son any "pick-onable" traits - just making him a glasses-wearing "nerd", for instance, but they went in a different direction.

I liked it, frankly. It's nice to see more of the human diaspora on TV.

Because then it wouldn't have been as interesting when Walt Jr. gets his own series about him running his own chain of breakfast restaurants and bed and breakfasts as a front and builds his own meth empire.

Breaking Bread.

Makes him more vulnerable, so that any ‘reprisals’ against Walt don’t just involve his kid getting killed, but a disabled kid.

I think the detail added to Walt’s, “Everything I did, I did it for my family.”

The disabled kid will always have a hard life. Kids picking on him, women troubles, self-esteem, inability to participate in activities (eg sports). The disability feeds the idea that Walt (more than any regular dad) would really want to improve his son’s life, considering all the kid had to live with. Walt did actively work to make his son happy with the drug money; see the expensive car he bought him.

Including people with disabilities is a step in the right direction.
These people DO exist and shouldn't be shunted to the side as being uninteresting and not worthy of attention.

It made Walt’s actions more admirable and Hank’s actions more villainous. Hank was willing to destroy the life of a disabled teenager so he could get another promotion, a character does not get more villainous than that.

It added more flavor to the story than hank's wife being a klepto, so I'd question the klepto part first.

I thought it just helped push the original need for Walt to make sure his Family was taken care of once he died.