It is possible, but discouraged, unless the child demonstrates exceptional ability in the subject. This is usually done with extreme caution and a meeting with the P is scheduled for such matters. If your child is keen, let his CL teacher / FT know, and they will advise based on your child’s performance.
As someone who studied basic syllabus for mother tongue in secondary school, I would not recommend moving back to the normal subject. The basic syllabus in secondary school is pegged to around the same standard as primary 6 and it would be very difficult to catch up. The B syllabus O level exam for example is mostly multiple choice questions, which is significantly easier than the normal syllabus exam. As long as your son does well in his other subjects, there shouldn’t be any problems with school admissions after secondary school.