I wouldn't do anything before talking to my boss in this situation. You're new - there might be internal politics that you're not in the loop on. So start there. "Hey Boss, wanted to check in on Larlo. I've struggling a bit with him - he doesn't appear to have the skills to complete XYZ and it's difficult to even get a meeting scheduled with him. Is there any history or back story here that I should know about?"
Assuming you get some version of "no" or "yeah, Larlo's a tough one" then I'd say "Okay. I'm going to work with him and see if we can get him on track, starting with a required weekly 1:1 meeting, getting him up to speed so he can complete X independently, and going from there."
After I talked to my boss, then I'd feel comfortable setting a firmer line with Larlo. "Hi Larlo - we're going to need to have a reliable weekly 1:1 meeting, unless you are taking PTO or it's a company holiday. What exactly are the possible conflicts with the time we discussed?" Do not take "well, we'll see" for an answer.
Then you jump in as manager. Take things slow, but start with baseline expectations (response time < 2 hrs during standard office hours, weekly 1:1, tasks getting completed) and go from there.
Document, document, document. Every missed meeting, every non-responded to slack, every task blown off. Be flexible and helpful as far as learning curve, but expect improvement and effort. Maybe he'll adjust and improve!
After a month or so, if there hasn't been some real improvement, then you go back to your Boss "Update on Larlo - I have not seen much improvement. He has blown off half our scheduled meetings with no warning, he's missed every deadline I've set for him, and is work is slapdash and unsatisfactory. What are the next steps on addressing his performance? I think it may be time to talk to HR about a PIP."
And then you go from there, through the company channels as recommended.