After over a year of no problems, I noticed today he can’t open one eye and it’s swollen

A hamster being unable to open its eyes may be due to eye inflammation caused by bacterial infection, trauma, allergy, or other reasons. You can clean around the hamster’s eyes with saline solution, then apply a little anti-inflammatory eye drops and a suitable amount of chlortetracycline or chloramphenicol eye ointment.

In addition, keep the diet light and avoid feeding the hamster too many nuts, mealworms, and other ‘heaty’ foods; you can feed more vegetables and mung beans. Finally, keep the environment clean to avoid the massive growth of bacteria, fungi, and mites.