As
@DCLane says, the current loan system is more a graduate tax, unless your son is particularly successful early in his career for most students there is little realistic prospect of repayment.
Assuming interest at 3% after 3 years of university, the student debt is around £60,000. On this basis earning £30k repayments would be around £300 per year and interest circa £2k. The good thing about plan 2 is that whilst repayment is unlikely, the monthly repayments themselves are also very low and whilst being low it's inevitably written off in 30 years.
On that basis, even if you felt able to fully fund the cost of university (tuition and cost of living) it doesn't make economical sense to do so, I would take the full loans.
Edit to add, I'm not a financial adviser, so take the advice accordingly. My back of the napkin calculations have assumed a 30k starting salary and inflation at an RPI of 2.5%, if your son starts his career with a much higher income, then the calculations may change, if RPI goes up significantly then this will also change the calculations somewhat. Broadly speaking however most graduates should consider it unlikely that repayment will occur, and moreover that even should their incomes rise over the life of the loan they will likely end up paying less in total than they borrowed if they take the full loans.
