Describe an arm without a forearm

Solution 1:

If I didn't mind bad puns, I'd call it 'foreshortened':

having the length reduced, curtailed or abridged.

(Definition from The Free Dictionary, American Heritage sense 2, paraphrased to suit the context.)

'Foreshortened' might be best if what is desired is a genteel tone, edging on discreet but with a possible slightly humorous doubletake on 'foreshortened by a forearm'. I mention the potential humor just so you're forewarned, because "forewarned is forearmed".

However, I'm stumped by the question:

reduced to a stump.

(Definition from The Free Dictionary, American Heritage, transitive verb sense 1, paraphrased to suit the context.)

Using 'stumped' might fit with a less genteel, more direct or colloquial tone. Nothing nuanced about stumped.

Solution 2:

I'd call it his stunted arm...

stunt - prevent from growing or developing properly (oxforddictionaries)

Here's a typical example usage from The New Yorker (1980)...

My mother took hold of his ankle, whitening her knuckles against his stunted leg. His left leg was short. It made every step a lunge.

Solution 3:

One could also call it a malformed or deformed arm. The former would emphasize that it had not developed properly, the latter that it had suffered some injury or environmental insult.