As to why his refund went down - what likely happened is that when he put in his income, TurboTax claimed the Canada Workers Benefit (CWB) (aka the Working Income Tax Benefit) for him. This is a refundable tax credit for people who work, but don’t make very much. However, students are usually not eligible for this credit, so when the tuition was added, the CWB was removed, lowering the refund. Other credits might be affected as well, as the tuition credit must be used before other credits, like the dividend credit
