What to do about an injured baby robin?

In my backyard there have been a nest of robins and recently all the babies have been flying away. Except one.

The “runt of the litter” wasn’t quite ready to fly but was forced to evacuate the nest due to an ant invasion. Unfortunately, some of the material that the robins used to make the nest was some sort of string/fishing line and it caught the baby robin’s foot leaving him dangling.

I eventually went ahead and cut him free (he was on a wooden board at this point that I had moved under him for support so that perhaps he or his parents could get him free, which they failed to do) and he’s been hopping around on the ground since with the leg that was caught dragging behind him and possibly with a damaged wing. So is there anything else I can do for the little guy? His parents are still hanging around and taking care of him but I feel it’s only a matter of time before they leave him.

Thank you for your help.