“CS junior at UofI/UIUC”
This is a VERY good university for CS. You do not need to attend a “top 10” university to do very well with a degree in CS, but since he is at a “top 5” CS program, and with “GPA 3.90+”, he is already doing really well!
“at top 5 programs only, mainly Stanford and CMU, otherwise goes to job market next summer”
I think that this is a very good plan. If I were in your son’s position I do not think that I would attend a lower ranked master’s program right now compared to UIUC. His chances are probably pretty good at getting into a top CS master’s program, and if he doesn’t then working for a couple of years and then reapplying to master’s programs would be a sensible plan.
When I was a master’s student at a well ranked university, there were certainly a lot of students (including me) who had some work experience. I do not know whether it was the majority, but it was certainly very common. I have always felt that having had some work experience helped me do better as a master’s student.
I think that your son is doing very well. I expect that you are proud of him!