Hi Emby,
The first thing I have to say is a normal urination schedule of only 2-3 times daily is not adequate. This means he is not drinking enough fluids on a regular basis, and this is something that needs to be corrected. 6-8 times per day is considered normal. I think with thals, fluid intake is critical, as there are many toxins in the body from the regular breakdown of red blood cells and the oxidative stresses that accompany thalassemia. In addition, when you are regularly on medication, especially chelators, plenty of fluids should be taken daily. Exjade does increase both thirst and urination, so this may be a factor.
As far as his current situation, is he experiencing any pain or burning with urination or any abnormal discoloration of the urine? Any pain in the kidney area? If there is no sign of infection in testing, and the doctor may choose to test again, and there is no reasonable explanation for the frequent urination, his sugar level will probably be investigated. However, if this was a problem, he would also be experiencing excessive thirst.
Please review the fluid intake of both boys. It is very important for anyone to get adequate hydration, but with the use of chelators, it becomes even more important.