First thing I think you should do is get rid of those rocks on the bottom of his tank, they can be ingested by accident and cause impaction issues. They also don't really promote any digging behavior which is something that tigers love to do. A better substrate would be to use moist coco fiber that is sold at many pet stores. They come in dehydrated bricks that you soak in water for a while to turn it into loose soil. It should be moist but now soaking wet. You shouldn't be able to wring more then a few drops of water out of it when you go to put it in the tank. Even though that is what the previous owner used and what he is used to doesn't mean that it is the best for him. He would probably be much more happy if he had some actual dirt to dig around in.
Also the type of food you are feeding him can be an issue. Freeze dried crickets are not good nutritionally and neither are mealworms. The best thing to feed tigers is live earthworms. If you can't get live earthworms then he should at least be fed live crickets that have been gutloaded and dusted with calcium powder. By gutloaded I mean the crickets should be fed before they are fed to your tiger. You can get special cricket food and gel to provided food and water for the crickets but a lot of people just feed them carrots or potatoes. Earthworms don't need to be gutloaded or dusted which is why they are the perfect food.
As for a temp gauge, you can pick those up at a pet store or even a hardware store. Digital ones are the better ones as they are more accurate. Tigers like it to be on the cooler side. Usually in the 60's to low 70's.
It is up to you however if you want to do these changes but I definitely hope that you will consider it so that your tiger can be happier and healthier, and you can have him as your pet for a lot longer.