Assuming his investments gained by 10% a year (which is extremely optimistic), he would have had to save $80k per year. Only a small minority of people have the required income to do that; especially at 25.
all I know is he started from scratch(no money passed from family). He started investing after college, after losing money to stocks for 1 or 2 years, he got into low cost index funds. Right now he is 37 and got about 1.5 mil in his account (mostly 401k and individual after tax). He started entry level position in a fortune 500 company (chemical) after college and have not changed job.