To access your grandson's prior-year return, you will need to make certain you are logged into the same account you did his taxes in for that specific year. It is possible to have more than one user ID linked to the same email address. It is also possible that you registered his TurboTax account for the prior-year with a different email address so make sure and check any that could've been used. If you used TurboTax CD/download software last year, then his return will be saved on your computer's hard drive wherever you chose to save it to and not online in his account. If you used TurboTax Free Edition last year, you will need to upgrade to Plus or pay upfront for the version you are using this year for him in the same account before you can access prior-year information from his Tax Timeline. If you used a paid version of TurboTax Online last year then you should still have access to download last year's return from his Tax Timeline. If you are just needing his 2015 AGI, there is no need to upgrade to Plus, you can get that from his Tax Timeline in the account he filed from last year.
https://ttlc.intuit.com/replies/5574744
You also always have an option to get his prior-year transcripts for free or pay for a copy of his return from the IRS:
https://www.irs.gov/uac/how-to-get-a-copy-of-your-prior-year-tax-information
