You can start your studies with general chemistry, or go straight on the menu of the program you will study chemistry.
But his doctorate is in biochemistry, probably go to study organic chemistry, known polymers, carbohydrates, free radicals, etc.
But as you mentioned theoretical chemistry, quantum chemistry then going to study and make use of computational tools in order to perform conformational analysis of molecules with
biological importance, interactions of molecules, processes under the effect of solvent, drug design, etc.
You can get an article titled: COMPUTERS IN THEORETICAL CHEMISTRY: TECHNICAL INFORMATION, author: Nelson Henrique Morgon.
Books: of Organic Chemistry, Volumes 1 and 2, authotr: Solomon. It also has the book of Organic Chemistry, authotr: Allinger.
Exploring Chemistry with Electronic Structre Methods by James B. Foresman & Aeleen Frish is the book for Theoretical chemistry who uses Gaussian 03W Programm.
As I see, we missed a classic one here. It is INTRODUCTION TO COMPUTATIONAL CHEMISTRY by Frank Jensen - at least if you are interested in computing and doing some quantum mechanical calculations (It covers a broad range from various electronic structure theories to geometry optimization techniques). Another good starter (especially if you are more interested in "theoretical concepts in chemistry" than in "performing some computations") is AN INTRODUCTION TO THEORETICAL CHEMISTRY by Jack Simons.
In order to do Theoretical Chemistry one must have the proper formation in Physical Chemistry as an undergraduate. This includes at least one full upper-level semester of Quantum Chemistry and another semester of Thermodynamics / Kinetics. In the expression, "Theoretical Chemistry", chemistry is the noun and theoretical is the adjective. If you used any of the "P-Chem in the Biological Sciences" textbooks, it definitely not enough. (Neither was the course that used it, but it can serve as a starting point.) I highly recommend Peter Atkins' very readable Physical Chemistry textbook, or his "Quanta, Matter, and Change" textbook. The earlier editions are just as good and are far cheaper. You should faithfully do all the exercises (There are Solutions Manuals.), then try the textbooks above, almost all of which assume that you have a proper formation in the Quantum part of Physical Chemistry. You will have a real sense of accomplishment if you acquire this basis. Have fun !