Hello MABS. I am a trichologist (specialist in hair and scalp problems), so I hope I might be able to reassure you. I'm sorry to hear your son's been going through a tough time.
Obviously without actually seeing him, it's not possible for me to make a firm diagnosis, but what you describe sounds almost certainly like a condition called alopecia areata. This is a very common problem, particularly amongst children, and I have seen it many times at the clinic where I work.
Just to clarify, the term 'alopecia' is just a general term for hair loss, and not a diagnosis (this is a very common misconception). The condition 'alopecia areata' is literally patchy hair loss, and is an auto-immune condition, where the body attacks the hair as foreign. In the majority of cases this happens due to stress, so it is highly likely that in your son's case this is due to the operation etc., even if he seems to be coping well.
This type of problem virtually always resolves itself, so I would urge you not to worry. Children tend to take these things in their stride, but sometimes in adults, the stress of the hair loss can create a vicious circle, so I would suggest trying not to make an issue of it to your son. The use of such treatments as steroid creams is not necessary, should this be suggested to you.
If you look at the bald patch, if the skin feels smooth, it is likely the hair loss has stopped, and the body is at a point to grow new hair. If you see small, stubbly hairs round the edge of the patch, which can be extracted easily, this is a sign the patch is enlarging. Please don't panic if you see these, as it may not get much bigger. Indeed, it is possible for hair to be regrowing whilst other hair is falling. Sometimes when the hair regrows, it comes back white, or pale, but usually returns to the original colour in time.
Make sure he continues to wash his hair as usual, ideally every day, as this will not cause more hair to fall out (other than normal daily shedding).
I hope this puts your mind at rest, do let me know if you have any other questions.