There are two problems with leaving the hook in his leg. First, if it snags on something, it will either tear tissue further or ensnare the swan on something where it possibly can drown, succumb to the weather or readily captured by a predator.

Second, and probably the most detrimental to the swan's health is the constant tearing of tissue and ensuing infection which can kill it.

The swan needs to be caught and taken to a veterinarian for hook removal and antibiotics. The Regal Swan