You are exactly right, those are signs of hyperthyroid disease and diabetes. These conditions are more likely in cats that are over 7 yrs old. Both of these conditions are very treatable and your kitty should be fine! =D
Hmm...has Dartagnan gained a lot of weight, that you can tell?
Diabetes can be caused by poor eating habits. Obesity creates diabetic cats, yes, but if he isn't gaining weight, then what you could do is you could monitor his daily activities such as eating, bathroom, and physical exercise. If any of these gets altered, then get him back on track by limiting/increasing whatever needs to be changed. If you have him on dry food, then obviously he will feel the need to drink a lot of water, especially now that he has decided he wants to grow a belly, right?
*Sigh* Cats.....they're all crazy!!!