Someone owed me money and he wanted to pay me in Bitcoin.
So I gave him my Bitcoin wallet address and he transferred the funds.
It's now complete.
But can he now withdraw the money since he has my bitcoin wallet address?
Since Bitcoin is deposited in your wallet no one can withdraw it while it is in your wallet. But if he gets access to your wallet, he can withdraw bitcoins from your wallet. Bitcoin wallets are usually given 12/24 words Seedphase, if someone steals them, they can withdraw your deposited bitcoins from your bitcoin wallet at any time.