Your son would not be responsible to the regulatory agencies / law enforcement in this instance.
These regulations are aimed to deter the owner of the real property (you) and the owner of the animals (your grandson), they do not usually deal with intermediaries (your son is acting as a tenant and sub lessor to the actual occupant).
This is different from the potential for civil liability - your son would be liable to you as the owner for the damages caused by the excess animals - but this is something that would be separate from the enforcement action.
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