Droopy eyelids can have a number of causes including lacerations, entropion (eyelids roll inward), ectropion (eyelids roll outward), trauma, tumors, disease such as Horner's Syndrome (see our article), eyelashes rubbing the cornea and so on. Since your gelding has had his yearly shots, it could be a reaction to them. It's best to have the condition examined by a vet so that proper diagnosis and treatment can be given.