Hi! Please make sure that he gets his meds and painkillers. I would recommend to use visco tear gel (it is basic tear fluid) several times a day to make the eyes more comfortable once your vet has assured that there is no irritation in the eye that is causing the streaming.
Keep him as comfortable and warm, but not hot. If his mobility is limited, please make sure that he is not sitting his excrements and pee for long. Clean/change where he is sitting all the time twice daily, give him gentle bum baths with hand warm water if he can no longer clean himself and check the underside of his paws regularly as he may struggle to take his weight off fully or partially; that makes him prone to developing foot infections.
Move hay, water and food close by if the struggles with getting around and switch from weighing once weekly to weighing once daily at the same time in the feeding cycle (like before his breakfast or dinner).
When piggies get older and frailer, it is often little niggles and a string of illnesses and complications that are starting to stack up as his immune system and his organs are not longer at full speed. Please be prepared to let him go when his time has come. If you have never had to deal with death before that is a frightening prospect, but we are here to support you through this.
Concentrate on giving Guinea good care, relief for discomfort and pain, but do not focus on extending his life beyond what his frail little body allows because of your own fears. Enjoy your special time with Guinea and make the best of it. Love surpasses time; you can create precious memories that last you a lifetime in a comparatively short space.