Here it means his sales were more than they were at the same time last year.

In this context, 'ahead' means 'better' or 'farther along'. It could also mean 'more' because more sales are better than less sales. Think of it like a race. The first runner is ahead of everyone else. This is a good term to use in sales because many times a company will chart the success of their salespeople on a graph. In this way, the first salesperson is ahead of the second, and the second salesperson is ahead of the third, etc. In your example, the salesperson is comparing his present sales record with that of last year.