I also think afternoon tea is a big rip off. My sister loves them, however, and so I wanted to buy one for herself and her husband for their wedding anniversary.
When I found out the price though, I balked at the idea and got them a voucher for a garden centre- where they could have used it towards the afternoon tea. They bought a plant instead.
That said, however, I feel that you should use it.
Take photos for your son to see what he bought with his money- or maybe even take him with you!
Whatever, just enjoy and, as others have said, be grateful that your son has been so generous.
Sometimes it isn't just the food, but the ambience that has the most value...