Hi,
It's a little hard to say without more detail, and of course some dogs are much more food-motivated than others, but all dogs generally like treats.

To find the cause of his picky manners, you must first eliminate any potential health problems and have him examined by your vet.

If he is given a clean bill of health and no problems are found with digestion or with his teeth (sometimes rescue dogs can have very bad teeth and eating is actually quite painful for them), you may need to assess the quality of the treats you are offering. Chicken, cheese, hamburger or steak are all great treats to most dogs. Most dogs like peanut butter. Some dogs even like carrots, frozen peas and apple slices (just be careful of the seeds, which contain arsenic). Packaged treats range in quality but it is essential that you look for treats that contain no artificial flavors, sugars, wheat or corn.

If he is still uninterested in really excellent treats, consider how much food he receives at regular meals and how much exercise he is getting. Increase his exercise and/or decrease the quantity of food he receives at mealtime (within reason - if you're not sure, check with your vet to work out a diet and nutrition plan). It may take couple days, or even weeks, but eventually most dogs will become more enthusiastic about treats.

If he's otherwise not a picky eater but seems reluctant to take treats from your hand, start off by throwing the treats on the floor for him. Depending on his past history, he may have a reason to be nervous about getting too close to human hands. You can gradually desensitize him to this until he is perfectly comfortable taking treats from you in any environment, but that's a different story!

Best wishes on your picky eater!
Megan