Final answer:
The best next step after controlling the intraocular pressure in a patient with advanced diabetic eye disease is to refer them to a retinal specialist for further evaluation and treatment.
Step-by-step explanation:
After controlling the intraocular pressure (IOP) of a 56-year-old man with diabetes, who presents with visual loss and symptoms of advanced glaucoma including iris neovascularization and proliferative diabetic retinopathy, the next step in management should be a referral to a retinal specialist for further evaluation and treatment (option 2). The specialist can provide a comprehensive approach to the patient's retinopathy and potential neovascular glaucoma, which may include procedures like laser photocoagulation or anti-VEGF injections, in conjunction with further glaucoma management.