Why does Atticus ask Tom to stand up?

It is to show that Tom was physically unable to do this to Mayella,

"Tom Robinson’s powerful shoulders rippled under his thin shirt. He rose to his feet and stood with his right hand on the back of his chair. He looked oddly off balance, but it was not from the way he was standing. His left arm was fully twelve inches shorter than his right, and hung dead at his side. It ended in a small shriveled hand, and from as far away as the balcony I could see that it was no use to him. "

Was he dying?

Nope. He would never die with a crippled left hand. It says above: "Tom Robinson's powerful shoulders rippled under his thin shirt. He rose to his feet and stood with his right hand on the back of his chair. He looked oddly of balance, but it was not from the way he was standing. His left arm was fully twelve inches shorter than his right, and hung dead at his side. It ended in a small shriveled hand, and from as far away as the balcony I could see, that it was no use of him." This explains what Tom looked like.

(P.S., this is for Aslan. I'm not copying your statement. I just wrote it down for reference just in case I forgot and yeah. I'm not a copy cater at all.)