I have to terminate someone's employment this week

I have a direct report I have decided to fire.

He had interviewed well and his resume was great. He's been here 4 months and he has not delivered on things in a timely manner. He has missed at least 15 deadlines entirely without any product, and others have had to complete it for him. The team is very nervous at this point. I've been very patient until recently, given that ot takes a long time to complete the Onboarding here, and he got through it all, but it's really getting terrible. He shared his whole screen on a recent 1:1, and I noticed he had 449 unread emails in his Outlook. I also know he has a side business, which he disclosed in the offer process, and is not a direct conflict with this work, but it seems like it may be part of the reason for the performance issue. I won't raise this in the goodbye meeting.

Also, the conversations I've had with him lead me to understand he really doesn't know the industry or work itself well enough in order to execute at all. Each product is filled with errors, he sends wrong documents, doesn't proof his correspondence, forgets tasks, has to be reminded, and even then promises "by Friday" and literally never meets this. Five team members have approached me with the same complaint. I've documented it all. It's exhausting, honestly.

HR is involved with me while we plan this. I will certainly listen to them.

I did not do a PIP because it is painfully clear he is unqualified. Performance can only improve if someone already has the skills and aptitude in the first place.

I'm practicing what to say to him in the meeting this week. I have understood from Reddit threads to not apologize, and to be very clear on the reasons. But I don't know how to say, "you simply don't have the skills and aptitude, or response time, that the position requires" because this sounds harsh to me.

Any advice on how to word this in order to make it clear that giving him a warning in order to make him improve would not have helped the situation?