Does he have an employment contract and on what terms? In some jurisdictions even no written contract can mean an implicit default contract, so you should consult a lawyer. And you may need to prove that you warned him about performance.
The other part are his 5%, but that usually is no hindrance for ending the employment. He will just stay on the cap sheet and can cash in later when you are successful. If that bothers you or your investor, you can try to make an offer (low, because he did not deliver). This is best to align with the investor and they make take those negotiations so you can focus on delivering.
Does he have an employment contract and on what terms? In some jurisdictions even no written contract can mean an implicit default contract, so you should consult a lawyer. And you may need to prove that you warned him about performance.
The other part are his 5%, but that usually is no hindrance for ending the employment. He will just stay on the cap sheet and can cash in later when you are successful. If that bothers you or your investor, you can try to make an offer (low, because he did not deliver). This is best to align with the investor and they make take those negotiations so you can focus on delivering.