Check his eyes.
I once rode a mare who was extremely spooky in bright light, but under dark conditions could be a child's trail horse.
If his eyes seem at all cloudy, then call a vet out to diagnose him. Most eye problems are treatable.
The most common eye problem is uveitis. It is quite painful, but can be treated by antibiotics.
It could also be cataracts or (hopefully not) cancer because he is an older horse.
The feed is also a possibility. Make sure you are feeding him something high in fat. All that protein is really not good for an old horse's kidneys and can cause founder. The beet pulp is an excellent way to keep weight on him already. Adding rice bran is another great way to keep weight on your guy, without all the protein.