Hemant,
Your doctor will evaluate your son, and let you know which "category" or level he falls into based on:
1. Age of the patient
2. Number of units transfused, which is in any case linked to the age
3. Iron levels in the body
4. Major Organ health
5. Splenectomy - done or not
The above is assuming that you have a HLA-matched donor.
Current data of BMTs done in India indicates that 94% of Category-1 patients have a successful recovery. The success rates drop off quite sharply for Category 2/3/4 patients.
I would strongly advise against a BMT if your son is not a category-1 candidate.
Cheers
Poirot