If a child is left with a disabled grandparent with mental issues, by a Father who sells drugs and mentally abuses the child, and a Mother who doesn't live there or care any more, wouldn't that be of interest to Social Services?

As long as they are notified, absolutely! They should step in and most likely place the child either with a fit family member ir with a fister family until/if the father gets cleaned up. I pray someone foes something because a child deserves much better than this. I woyld open my home to this child if i were able. Thoughts and orayers to u for caring and for the child.

Yes. If DSS doesn't know then they need to.

Yes hopefully they would already be involved.