From the outside looking in, this seems to be a problem of your own devising. Ie, it’s not that you’re bad at interviews, it’s that you believe you’re bad at interviews and this then becomes self-fulfilling.
Everyone (who isn’t a sociopath) is nervous at interviews. It’s an unknown quantity, you don’t know what the interviewers are going to be like, whether you can cut the mustard, your potential future career depends on whether you can impress in the next 20 minutes, etc etc. But.
You know this guy, he knows you. You’ve worked together previously. You’re ahead of the curve because if he didn’t think you were suitable for the role then you wouldn’t be in the chair in the first place. What if you had the job and this were a team meeting, would you be “useless” then? Ask yourself, what’s the difference?
In your head it’s an INTERVIEW – it’s not, it’s a chat with a work colleague. Best-case scenario, you get offered a better job; worst-case, nothing changes from where you are now.