If he qualifies as your dependent, meaning he is not providing more than half of his own support, since he is paying for his own college, you would claim the 1098T on your return. The education expenses when it comes to claiming the credits are treated as being paid by whoever is claiming the dependent even if the dependent is paying their own tuition.
A dependent cannot claim any education credits on their return.
Even if you don't claim him but could, he would only qualify for the nonrefundable portion of the AOTC. The IRS does now allow someone who could be claimed as a dependent to claim the refundable part of the American Opportunity Credit.
Yes, if you did not enter the 1098-T on your return you can amend your return to claim it