The best thing you can do is be fair. Which is very hard in a situation like this.
Let me stress this first, mind you this is solely my opinion: one of the worst things an employer can do is just up and fire someone who was never officially given a heads-up that something was amiss. Not even given a chance to do better. Barring, of course, extreme circumstances. And I know that HR jerkwads do it all the time. Doesn’t make it fair. And, if you have done that already with this guy - good on you, that makes the prospect of termination easier to stomach.
To me the fairest thing would be this: hire the new guy. Let him work thru his probation period if you do that, otherwise just make sure he does right for a couple months. Then let your slacker know what’s up. Tell him, professionally, where you need him to improve. Try to avoid comparing the two, to avoid any bad blood between them (or at least at this stage, it’s probably inevitable). If he gets upset and pissed off, walk him out at that point. But, some people don’t realize how poor they’re doing until it’s laid out there plain to them.
From a different viewpoint it seems stupid to not realize it, but you never know their upbringing or previous work history; they may not know. If you straight up tell him shape up or ship out, and how, and he doesn’t- it’s on him.
Anyway! The ideal situation is, within these two-three months that the new guy is doing his thing, perhaps you can find work for your slacker. If it’s cheesy production work or, if it’s sawing, staging, packaging, that kind of thing. Something that doesn’t take as much thought or care. Then you can keep both guys and you’re better for it, best case scenario.