This also happened to me. I interviewed someone from an IT staffing agency. Seemed perfect. A different person showed up for the job, and he knew next to nothing about IT.
https://www.weightwatchers.com works really well. I thought it was just a brand of frozen food, but it's not. You get a set number of "points" each day, which more or less equate to calories. It makes counting calories much, much easier. Worth a look.