First of all, prayers for your DS's friend and his family (and your DS too - it's hard to watch a friend struggle with serious illness, especially when you're a teen and think you'll live forever).
Depending on what type leukemia he has, there is up to a 70% or better survival rate, so it's far from a death sentence.
Bravo foryour DS and his friends for visiting and supporting their friend. Sometimes people are scared for whatever reason. They don't know what to say/do, they don't like hospitals, whatever. So just being there is fantastic. The boy won't feel like his friends are going to "move on" without him, if that makes sense.
How long they should stay depends on how the boy is feeling. If he's already had surgery to implant a port for chemo and gotten his first round, he may not feel up to them hanging around for long, but it's actually likely that it hasn't happened yet, and he may feel totally fine - just the same as before he got the news. It's weird to be told you're *very* sick when you don't *feel* very sick.
What to bring him depends on what he likes.

What would your DS give him as a birthday gift? Something similar would probably be nice. I'd stay away from food items, though, until you see how he's feeling. Also, there's nothing wrong with asking HIM what he'd like. Just have your DS say, "Hey, what would you like next time we come?" or "What do you wish you had here in your room that you haven't got?" Teen boys usually aren't shy about answering truthfully. If he wants a basket filled with M&Ms or whatever, you can get it for him next time.
Other advice - he will spend much more time at home than in the hospital. Just make sure they don't forget about him when planning stuff like trips to the mall, ball games, whatever they normally do. (Not saying they would, but if he couldn't make one game because he was sick, they might assume he won't be able to come to the next one either. Even if he can't go, it's still nice to be asked.) I've seen some teens shave their heads as a measure of support when a friend loses their hair.
But remember he's still the same guy. He wants to STAY that same guy, he doesn't want to be "that guy with cancer." So you don't have to go overboard with gifts. Time is the best gift, even if it's just a phone call.
That's great that your DS and his friends are going to visit. They shouldn't be afraid to talk about the diagnosis. It doesn't have to be the "elephant in the room" that everyone can see but no one dares to mention. It's okay to ask him how many months of chemo he'll need, will he still be able to go to school, etc. They can/should also talk about all the normal things they would talk about if they were just hanging out at your house.
Hopefully in several months, we'll see a post from you asking for ideas for an "I'm done with chemo" party.

(but it won't be an easy road between now and then. Prayer is the most important thing of all.)