Hi,
Welcome to HCM.
Since the bubble is causing no problem and since it is not increasing in size, I think it can be left alone. What is more important is maintaining good hygiene by regularly washing the area especially the groove behind the head of the penis, by pulling back the foreskin. Regular washing may make the bubble subside. If it is persisting consult an Urologist and let him examine the lesion and manage it accordingly.
Wishing your son all the best.

Dear Mr anonymous, good that you have noted the problem so early. There are 2 possibilities with it. one possibility is that it is a kind of a cyst under the foreskin or over the glans. If so, it has chances of getting infected sooner. This is a less common situation. In second possibility, often bubble under foreskin, is seen while passing urine especially in case of prepucial stenosis. since the foreskin hole may be smaller its likely that skin cant be rolled back, and in such situation when urine comes at a speed.. it halts at foreskin opening and foreskin bulges like a bubble, this situation needs circumcision as it is likely to cause back pressure kidney damage.
The different opinions by different doctors is mainly because, you have not given a clear idea of what child is suffering from, a picture of the so called bubble felt by you may clarify matter.
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