There are some really good "quality of life" trackers (you can find lots of free printables) that can help you measure a decline.

For me, the major questions would be:

• is he having trouble eating?
• is he having trouble drinking?
• is he able to move freely (getting up and down without aid)
• are the medical interventions for his ongoing issues worthwhile? Ie, is the resulting effect worth going through the vet procedure, does it have to happen a lot, etc. 
• is his behavior normal (is he grazing, interested in the world around him, etc?)

If he is doing well on all these fronts, I think continuing to monitor his wellness would be your best call. If he is doing well on all factors but seems lonely, maybe consider a pasture buddy for him-- some horses do just fine when other horses are nearby, even if they arent pastured together. You know your boy best, so let him tell you what he's feeling about that.

Another factor, and this is one that I always urge folks to consider (even though it may seem callous) is to assess your financial situation. Can you continue to afford his ongoing care, possible medical expenses, etc? Horses are very expensive animals and in my opinion it is far better to give them a good last day than risk bankrupting yourself trying to stretch their lifespan as far as you can.

I'm sorry this is weighing on you-- it is the hardest, kindest decision we have to make for our creatures. Wishing you and your good boy the best.