Thank you for your question.
I am Dr. Su S. and I will try my best to help you.
The bruise is due to extravasation of blood.
This may take some time to get absorbed.
Some children get easy bruising, often the cause is not found.
Children get it more easily from any trivial trauma during play.
Bruising is worrisome when it is accompanied by other symptoms like bleeding from nose, gut or with urine, frequent large ones, swelling of lymph glands etc.
You can observe it.
If it increases in size or is accompanied by other symptoms you should take him to doctor.
Doctor can check blood for platelets, coagulation factors, bleeding disorder etc.
Vitamin deficiency can be a cause.
Please give him fresh fruit, vegetables abundantly.
I hope this information will be helpful.
If you have any query, please feel free to ask.
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Wish his healthy life.