Emby,
in addition to what Andy and Waleed has said above ,, i want say that,,
1) volume of blood (RBCs) is calculated to the weight of the patient,,, as far i know 10-15ml/kg is the normal blood requirement for a thal ,,, as your son is in growning age and would be gaining weight so it could be a reason behind decrease in gap of Tx interval and increased blood requirement....
2) another reason i witnessed here in the thal centers of my city is the mixing Normal Saline in blood to make 2 blood bags out of one to overcome blood shortage ,,, tha t means less RBCs in a blood bag than a normally transfused blood bag ,,, that means it is not the accelerated hemolysis behind this ,, it is less increase in hb than a normally transfused blood bag ... i dont know about the situation at your place but now its a common practice here ....
Ameen ...