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There two or three possibilities for the condition that you have described.
1. Phimosis: It is the inability to retract the prepuce. It causes urine to collect between the preputial skin and the glans
of the penis and hence the swelling appears. The urine may be clear but it may be causing him irritation and difficulty while passing urine.
2. Insect bite: sometimes an insect bite can also cause a swelling over the prepuce. More than the bite it the allergy
to the sting. But it is unlikely to stay for these many days and usually does not cause a lot of discomfort
3. Balanoposthitis: It is the inflammation of the glans and prepuce due to an infection. It usually is accompanied by fever
and some dirty looking discharge. So I feel it to be less likely.
For all these conditions you should consult a pediatric surgeon. If it phimosis then it may also require surgery. But I can not dram an opinion without having a look at your son. So Consult a pedistric surgeon near you for a proper diagnosis and proper treatment
Hope that helped
God bless you