List objective requirements he cannot meet for your lead/senior positions
Talk about "years of experience" required to become a tech lead. This question has tons of answers about why years of experience matter.
There are a lot of things that someone with a year of mediocre experience will not have.
When you are discussing the requirements for your higher level positions such as tech lead, it's easy to describe the types of skills/experiences required even if just "years of experience."
How to point out gaps if he doesn't believe you..
If the candidate protests, there are a few things you might want to consider. First they are pretty confident in their naive background. This is not necessarily something you want in an employee.
Second, you can suggest asking questions from those interviews. Make sure to say something like, "I really don't think you have the right experiences for a tech lead/senior position, but if you want we can go through our interview questions anyways."
Keep in mind that it'd be easy to ask questions which are nearly impossible to answer with a year of mediocre experience:
- What lessons did you learn from mistakes you made initially but only become obvious after a few years with the product?
- When starting multi-year projects, what approaches do you take towards gathering requirements?
- What times have you actively guided other employees and mentored them technically? What were the results?
It'll become really obvious to the candidate that they don't have the experience needed to answer your questions for the position.
Should you have him interview with current leads/seniors?
Just a note, I would avoid interviewing with your current tech leads for a tech lead type of position because they will obviously find this person not qualified. You don't want the new hire to immediately have resentment or negative attitudes from your current seniors/tech leads..
If you want to do this be very careful to not make him feel mocked/etc. Keep in mind he is interviewing for your company, too, and a terrible experience interviewing might not go over so well.