No, co-signer can’t access your credit history
Lenders are not allowed to disclose YOUR information with anyone, not even a co-signer.
Regardless – you really should disclose it to him yourself, even at the cost of him backing off. Co-signers take a huge risk on themselves when they co-sign. You can mess HIS credit very easily, and so you are expected not to hide the truth from him.
In fact, I advice people to NEVER co-sign, or at least sign only after they’ve seen the credit reports of person they’re co-signing to, and are fully aware or the risk.
Any late payment will spill directly to the co-signer’s credit report as well as to yours.
Co-signers are usually NOT informed on late payments until the account goes 90 days past due, by which time major damage is done to the co-signer’s credit.
If you face a problem meeting payments, make sure to alert your co-signer immediately, BEFORE any damage is done.