There's another thread here about hind quarters atrophying - I'll see if I can find it. Unfortunately, you have to decide what's best for your cat. 12 is not so old - many cats can live to be 17, 18, at least... If your cat has enough recurring problems that you think he's finished with tests, than maybe it's time, but keep in mind that, if he's otherwise healthy, this may be something very correctable (like a blockage or something). Cats are notorious for hiding their discomfort and pain.
The ultrasound is probably worth it - even if your kitty does have cancer, at least then you'll know. The alternative is that you just wait it out but the weight loss is a sign that something is definitely wrong - your kitty will almost certainly get worse, not better.
Storm (beeps and has a pink nose and toes),
Misty (with big blue eyes and tasseled ears),
Anya (only ever called Honey - she's that sweet),
Zander (who goes by Punkin' cause he's almost that bright)
Kasee (couch potato who thinks she's a dog)