My grandfather helped sign for me a $10,000 Private Loan through Sallie Mae.
He is just the CoSigner, and I am the student.

Will this loan show up on my grandfathers credit report keeping him from getting other loans, etc.?
Or will it only show up "IF" I ever don't make the payments, or will it go on his credit once the repayment cycle begins?

Thanks a ton.

The loan will show up on his credit reports, since he is ultimately responsible if you default.

I'm not sure how it works as far as DTI goes. I think if he can prove that you've been making payments on the loan, then he's off the hook as far as DTI is concerned, but don't quote me on that. If the loan is not currently in repayment, it will show "deferred" on the credit report and shouldn't be calculated in DTI.