Failed BM in SPM, can I still further study or not?

Don't panic la. My cousin also fail BM and he still study diploma in a private college, no problem one. But if you target IPTA or matrikulasi, credit in BM memang wajib for most courses, and to get the SPM sijil recognised properly you should pass BM. Best thing your son can do is repeat BM as a private candidate (calon persendirian) in the November SPM ulangan. Register through the state education department, exam only that one subject, cheap and fast. My cousin took ulangan, passed, then applied diploma no issue. Cert with all subjects intact opens more doors later, especially for gov jobs and IPTA transfer. So repeat lah, don't give up the whole future for one subject.