Expense checks to avoid taxes?

I worked for a small construction company for about 6 months last year. I was salaried, and on my first day the owner explained to me how he paid employees on salary. Two thirds of my income was paid normally with taxes withheld. The remaining third was paid around the first of the month via a paper "expense check" to "help us both a little with taxes". This money was never used for work related expenses as any expenses I came across were either reimbursed on my normal paycheck or paid for with a company credit card. I questioned the legality of it but I needed the job so I didn't ask too many questions. Now it's time to file my taxes and looking at my W2 from there, it obviously shows much less than what I actually made.

I'm assuming I do actually have to report the other third of my income not shown on the W2? Should I report this so future employees of his don't run into the same issue?