This is one of those areas where daycare is a big help. Daycare kids - particularly 'center' daycare with 40-50 kids of different ages in different classes running about at all times - in my experience can sleep through anything. My two both had absolutely no problems sleeping with each other (and they're in a room together, and have been since the younger was about 4-5 months old; before that the younger was in our room). If you can sleep at daycare, you learn to sleep through anything.
For kids not in daycare, the noise is an issue. Can they sleep when you vacuum? Pets crawling on their bed? If he's okay with those sorts of things, he'll be okay with the baby.
I would generally say that the first ~30 minutes of sleep is key to your oldest sleeping through baby waking up later on. In the first 15-30 minutes you wake much more easily, typically; after 30 minutes usually you hit deeper sleep and can sleep through more noise. For that, try to time your oldest's and youngest's bedtimes so that they're not quite simultaneous; the youngest should go to bed first and the oldest second, by 30 minutes or so. Especially true if the oldest is a problem bedtime child like mine (ie, will cry or complain). This 30 minute difference actually works out pretty well, in our experience at least: put youngest baby to bed, then start bedtime routine with youngest - probably will take about 30 minutes, right? (If it takes more than 30 minutes to do the routine, possibly have one parent start early.)
That said, it varies by child how easily they wake up. If your eldest will wake up to relatively little noise, or regularly wakes up at night, you will need to determine what the 'come to Mommy's bed' policy is. That definitely increased when our oldest was about 3; not sure if that's age related or younger brother related. I don't think it was due to being sad about younger brother and attention, it seemed to be solely based on waking from dreams or nightmares. It's mostly stopped now (by 3 1/2).
Overall, most of what you do is going to be based on what happens; you can't really effectively prepare for everything because you don't know how the two will interact, nor how each will sleep necessarily. Other than some white noise or music to sleep to (which is a good idea regardless of baby!), or perhaps trying to vacuum more, it's unlikely anything you do in the last couple of months will really make much of a difference, physiologically.
Instead, talk to your son about what's going to happen, and perhaps talk through the bedtime routine. He's old enough to understand most of what you tell him. Let him ask you questions and encourage them. Getting him on board with the changes will do more than anything else.