His pending derivative case gives him nothing to travel on. Going ahead of you would require his own valid basis for entry and study, granted on its own merits. His inclusion here still depends on being unmarried and under 21 at the point the rules measure, and on your case succeeding.
Two separate questions: his study plan and his derivative place
A separate study plan requires the appropriate admission category and supporting documents; whether a visa is required depends on the person’s circumstances. Student status does not automatically become permanent residence. Also check the rule for entry as a derivative immigrant: the child cannot simply activate derivative residence before the principal. Review any age-protection issue and the actual travel order before booking.
The FAQ recommends initial inclusion of the eligible spouse and unmarried children under 21 who intend to join. Only a spouse and unmarried children under 21 can be included as accompanying family, each with their own processing fee and gift per the official page.