Absolutely he can. My son did something similar. His high school doesn’t allow students to take more than one math class at a time, and he wanted to take Statistics in addition to his other math. So he signed up for an online course at a local 4-year college that allowed kids to enroll if there were extra spaces available. The class did not count towards his high school diploma, but he included it on all of his college applications (and sent a transcript from the college to the university at which he eventually enrolled.)