Yes, he can enroll now. When the time comes and you apply for your parents sponsorship, he needs to be enrolled in university/college ever since attaining the age of 22, so since he's still 21, that's ok. When the time comes he'll need proof of continuing studies and that he's been fully dependent on your parents.
Good luck, and don't lose hope! If you applied now, most likely it would take up to 4 years for them to start working on your process, so...

Who knows when they resume accepting new applications it will be faster?