MY cat, Benji (how do I post a photo?) went blind from High Blood pressure. He WAS getting around pretty good but lately he is not. He seems to want to go more places and then falls off sidewalks and bumps into things.
Just because he doesn't put his tail up doesn't mean he's not happy. My two, Book and Neelix do always have their tail up but my MowMow (died in January at 16 years old) NEVER put his tail up. He purred, loved on me, was a wonderfully content cat. He just wasn't one for sticking his tail in the air.
My newest addition Jack Jack never puts his tail up either. He's a happy boy who purrs and plays and loves to cuddle, he's just not for sticking his tail up.
Only you know what will make your kitty happy. Give him plenty of attention and affection and it will all fall into place. Be very attentive to his needs and everything will be fine with him. Be there for him. Make sure he is comfortable with his surroundings.
I agree with the other posters - just because he doesn't put his tail up it doesn't mean he's not happy. As long as he gets plenty of cuddles and attention along with his food he'll be a happy kit. I often find a couple of treats don't go a miss either!