I feel like the OP is saying he didn't even know he had a son let alone one old enough to give him a grandson or two.

If I understand correctly, OP didn't even know that his son existed, maybe that was mutual. As in: the son only just found out that OP was their father. It's reasonable to ask a parebt for help.

But it's not unreasonable to refuse. Like people are already saying, not only did no one tell him he even had a son, but he went so long that his son grew up, got married and had kids. Not only is he unfamiliar with them, but now they're just asking for favors out of nowhere simply because of blood ties. Sorry, but blood relation does not obligate you to do anything for those you're related to and if he simply did it, he'd be acting the doormat.

It might not be the sons fault, he might have been looking for his father his entire life and wants nothing more than to include him in his family.

Yes? Hence me pointing out that the son never told the dad he existed?

There's a difference between "Hi, I'm your son and want to have a relationship with you" and "hi, I'm your son and I need a babysitter." Even if the son suddenly found out about this father after reaching adulthood, having enough time to get married and have a baby...that still makes it weird behavior. Who thinks "wow, a dad I never knew about and know nothing about and have no relationship with. Better hand over my eldest child immediately."

Longing to have a relationship is different from 'do me a favor." Foolish to assume that one means the other.

Exactly

He could be 36 and be a grandfather if he had his son at 18 and his son had his kid at 18.