Can you elaborate a bit more on the "forced him to confess" part? What did he say/do that made you think he was hiding something? I'm asking to get a better picture of the dynamic between you two. 

In any case, 300 is a very strange number. Humans are not good at remembering amounts of occurrences; I'd think the number would start to blur when you get past the first 10 or 20. Past that most people would need to keep track through other measures or keep a 'schedule' of sorts. (This many girls per month x 12 x number of years). He could just be guessing. In any case, whatever the number truly is, it will be high.

Did you ask him why he slept with so many people? And why he stopped now? If you want to try move past this I think it's important for you to know that his past truly stays in the past.