@Wjp007
First, just in general about what you said regarding the stats: was his courseload very difficult? filled with aps & honors? if not, a student with a very heavy courseload would be preferred even with a few Bs. Weighted is a better indicator than unweighted for this reason.
Also, low 1500s SAT probably won’t count much in his favor: it won’t put him out of the running, but it’s not a boost.
Now then, for specifically UChicago:
Essays WILL make or break the application, unless the applicants credentials are completely world class.
UChicago’s uncommon essay is a major factor in whether or not a student gets accepted, and even the other two (Common App & Why UChicago).
For almost all top tier colleges, a stellar student with mediocre essays will easily get passed over in favor of a mediocre student with stellar essays.
For UChicago in particular, the essays can be considered the majority of the application. The stats get you considered, the essays get you in.