The type of MMA or the gene defect is not mentioned here.
The problem your son is facing may be due to the Carnitor
in the form of over dosage or he may be allergic to it. You can do one thing that you can stop carnitor for some days with due observation under your childs physician. No worry you can include the food items which are rich in producing Biotin
and B12 which is most required by your son.
However, there is no need of Sizodon as such as per my understanding whatever information you have provided here.
The above mentioned things should be confirmed by the clinical as well as lab investigations by a Registered Medical Practitioner Specialist in the area.
There are many herbs available for the treatment of such type you can also think for the same... Especially Karnapuran, Nasya and Basti like treatment will help your child much in a natural way of growing.
Dr. Jitendra V Suralkar