So your son won't answer publicly, but here's my perspective:
It's not about penalizing him, or anyone else who are wealthy.
I don't consider myself rich - I live in a normal terraced house in a London suburb the same size as my decidedly working class neighbors, and I don't have huge piles of cash anywhere, though I do have some investments
But I'm well enough off to have it nice and comfortable, and I make several times what most of my neighbors do with according benefits in terms of how I lead my life.
Yet despite paying significantly higher taxes (about 10% points more than someone on an average UK salary), the net effect of the reduction in my spending ability from taxes is far less than the effect on my neighbors.
I won't even notice a 1% difference in my tax rate. I don't even know how much I spend on groceries each month - it doesn't matter and isn't worth my attention; I waste more than 1% each month on stuff from Amazon I probably don't really need. A 5% difference would be noticeable, but not painful.
For my neighbors, a 1% difference can easily mean the difference between making rent next month, or wait longer before fixing the car they depend on to get to work, or having to count the coins and watch every penny to even be able to make the paycheck last the whole month. A 5% tax hike for most of my neighbors would mean they'd be in big, serious trouble.
This is one large reason why many people believe higher taxes on top earners is justified. Whether in the US or UK, or most other places, the current tax bands most places means the burden of tax in terms of the effect it actually has on peoples lives is far greater on those who makes least to start with.
Yet people argued the economy would collapse with a new 10% higher tax band for people making even far more than I do..
Any argument about what's "fair" is arbitrary and based on personal value systems anyway. Why is a specific function of income to tax rate more fair than another? It boils down to your values. Personally I don't consider it fair that many of those who take all the shitty jobs I don't want to do, and who struggle to keep things afloat, carry a heavier burden than me. After all - without a well functioning society around me, there would have been no basis for me to achieve the relative wealth I do have.
In another vein, I'd also prefer to have less misery around me. I can pay to move somewhere where it's better hidden behind fences, or I can support changes that actually helps make life better for others, whether in terms of the tax system or others. The latter seems like a better investment.