Laz - if I may add a little wrinkle to your scenario -
I think your son is better off putting abotu 60K on his move up and the 20K in the bank for cushion. The reason being - too much available and ready cash is a big temptation to spend on things that you want and don't need.
Let me tell you a little story. When I was 19 years old, I bot a little house with about $5,000 I had saved. Less than a year, I sold it for a profit of approx $100,000. I decided to put $90,000 in the bank and invested $10,000 in another house. Well, family started to borrow money and never paid back, some of it I invested in the stock market and lost it. Bottom line is that when I sold the house four years later for $50,000 profit - that is all I had. Had I bot my house with the cash instead of putting in the bank, I would have walked away with $150,000 instead of $50,000.