1. How's the tension throughout the piece?

It was very tense in the beginning, starting out right in middle of the action. The way you set up the breaks in Grandfather's dialogue to add Toby's thoughts kept it going. So, good tension through the beginning, but it slacks off a little in the end, where Toby is going to bed and his Grandfather is trying to be kind.

2. Do i need to include more description of the characters/settings?

Personally, I like to know every iota of information about a character, but this story had good description and was satisfying. For example:

"...images of Grandfather’s face flashed in my eyes; his squinting eyes, red cheeks glazed with sweat, and the bulging vein that pulsed in his neck."

I really liked that. It gave me a clear description of the man we're dealing with.

3. Other comments?

The story is very good, but what takes away from it a little is why Grandfather wants Toby to swallow those mind-numbing pills. If it is an excerpt from a larger piece, I can understand, but I was a little confused about that.

All that being said, great piece of work, I really enjoyed reading it.