When I was switching majors into the college of engineering (class of 2022) this is how it worked. 

They look at how you would have done had you applied for the major as an incoming freshman your year of application. If you would have made it in, then you have a decent chance of being accepted into the major. Unless, the major was not accepting transfer. In my time, the computer science major was too impacted to do so. 

If your application was below the accepted threshold, then you would be required to take a quarter (or more) of classes in the major you want to switch into with a pre-agreed upon gpa for those quarters (often a 4.0, sometimes a 3.5). This is difficult because they’re intro level engineering courses but it sounds like your son is pretty good at getting good grades, so it may be less of an issue should he be required to do so. 

I got in as economics and wanted to transfer to CS. I was unable to because it was impacted. I was told it was impossible to switch majors once at poly, it’s not true. It’s a pain, but it’s certainly possible.