This was closed as a duplicate, which I think it's not. Because Germany differs a lot from the default (US) here. We don't have a sick day allowance. Cheating on sick days is a firing offense. And while that might mean nothing in a country where you have at-will-employment, in Germany it's the difference between 2 months pay, unemployment benefits and a severance package on top and... standing on the street without a job and without any unemployment benefits at all.
Also, if he does become the team lead, "just let it slip" is unsound advice. Once you did, you have legal problems if you want to fire him for it later. With a firing offense, you actually have to use it when you get to know it. You cannot just say "you did this for years and now I'm sick of it."
Basically, his question "We live and work in Germany. Is there a way of firing him?" is not answered by the duplicate at all.
So please reopen it so we can craft an answer for this question that takes germany into account.