I think it is reassuring that your DS will be taking both courses at the e-school and the e-school thinks it is fine. Taking both at the same time may be problematic only in a system, which integrates coverage of trigonometry in one course and then presumes that knowledge in the next course. The only other concern is that the writing demands and project schedules don't overload/overlap too much but again there is generally more flexibility with an e-school.