Your son does need to enter that T2202A Tuition slip on his tax return. He may be losing the $300 of his refund as the Working Income Tax Benefit (WITB) was being applied prior to entering the T2202A, and he is no longer eligible for this credit if he is considered to be a "full-time student" with four or more education months.
In order for you to claim any of his tuition, it must be entered on his return first, and then there will be an "amount available to transfer" shown in the Student Summary which you can use. Here's some more information in transferring and carry forward tuition: https://turbotax.community.intuit.ca/replies/2647970
