Assuming this is legal and you and your boss have already verified this...
What to learn in the interview
Do you have any suggestion on how to approach his interview
You want to figure out a very clear explanation as to why he wants to work for free. A senior (24+ years experience) tech professional not having any paying options seems really weird to me.
If the explanation doesn't make sense, don't hire him. Without any other details, my guess is this person knows that getting a job when you are employed (vs unemployed) is far easier and is going for the "easy" way to get employed.
Also look at whatever resume gaps exist for the employee and try to understand them. They may add context to the above.
eventually how to push back on my manager if he really really wants to hire this guy no matter what?
Six months is a long time to work without a paycheck.
How much money will your company spend investing in him (training? lost productivity from other people? actual training? technology costs? Etc). Just because he is not being paid a paycheck does not make him free.
Your boss should understand this. If not, talk through the still actual costs of the employee.
Talk through, "what would happen if he gets a paying offer elsewhere while working for free here?" and the answer is probably - leave, immediately.
Mention that someone working for free will probably be less dedicated than your current team. This can affect others the free-hire is working with and bring their productivity down. "I'm not getting paid, who cares if we take an extra long lunch break every day and talk instead of work?" etc.
Also, I would strongly encourage you to talk with your legal department or HR to fully understand legal implications of content produced by this person, what liabilities you might incur (can he sue you for back wages?), etc. You want to have a perfect understanding of what the implications of this are, given your local employment law, etc. Check out this link as a relevant source for the UK.
My read on the situation is he's desperate for anything and hasn't had any luck, so is trying to work for free. I'd also say that it seems likely this person isn't that great to begin with or they'd have been able to get a paying job somewhere already.
Also, read this question for some good insight.