Sharmin,
While you should remind him that some of his behavior is not appropriate, please keep in mind that these various medications can have side effects and he may not be in full control of his moods. Even dropping the prednisone dose can have some short term effects, as his body adjusts to the lower dose. The psychological effects of everything he's been through lately may also be partially responsible. Whereas he has felt fairly normal with his regular routines in the past, the trip to California to see a special doctor, the very harsh IVIG treatment, the ritux, the drop in prednisone dosage, and the various physical reactions can all play a part on how he is feeling, and not feeling as he normally has, can all be somewhat overwhelming for anyone. Patience from the parents and reminders to the child about his behavior are important. I would not threaten punishment or restriction in activities to get him to behave at this point, as with what he has been through, it's easy to see why he would be acting up. He may have some fears about his own future right now and reassurance that he will be fine and that his doctors are doing something over a short period so that he can be free of a long term problem, is in order. It must be very hard for a young boy to endure all of this, so intervene when necessary but keep your composure no matter how much he acts up. Seeing that his parents are confident about his future and understanding about his present, can help him achieve the confidence he needs right now.
One thing I think you should also consider right now is special stress on a very nutritious diet and make sure he is drinking plenty of water. A special treat here and there is fine, but try to stick with a diet that can restore his body from the effects of the various drugs.