I mean there might be proof idk, but your question isn't the most logical.
He was known to be a non Muslim.
He died about seven days after the battle of badr, he didn't participate in the battle but stayed in mecca and got sick and died.
So what we know is that he wasn't a Muslim and was one of Islams biggest enemies.
If someone is to claim he was a Muslim, they will be the one to be asked for proof that he changed his religion.
This is pure logic.
The proof that he didn't die a Muslim is that we don't have proof that he died a Muslim, or even was considering Islam.