I too have a mantis which had one arm missing from a mismolt, but as it happened at L3 and he is now L6, his arm has grown back, gradually with each molt. He could catch food with his other arm always. If your mantis never grew an arm, it might be from the beginning and have some genetic reason (like people or animals who are born missing limbs - just my speculation)
They can regenerate, but it depends on the amount of damage and also the number of molts left. Like what Hibiscusmile said, the mantid won't be able to regenerate the forelegs since it has about 1 molt left. But even then, it'll still make a fine breeding specimen (if the female is receptive of course).
He'll be your special little guy.
I still have an Anolis equestris that I found with a smashed head a few months ago. I am surprised that it lived past a few days but now it looks pretty good, but with only one eye. I have to hand feed it every time, because it just can't or won't catch the prey I have put in with it. I have found out that they can live for 16 years, so I might have alot of hand feeding ahead of me, but its my buddy now. ^_^
