Verifying Gmail account in Mac Internet Accounts failing

I am trying to add my brother's gmail account to his new Mac, which I have done in the Internet Accounts section successfully with the right password. However, he is unable to receive mail in the Mail app, and the Connection Doctor says that "Trying to log in to this IMAP account failed. Verify that the username and password are correct." I've deleted the account from Internet Accounts and re-added it a couple of times as well as restarted his computer, but nothing's worked. Any thoughts?