Thank you for your question.
I am Dr. Su S. and I will try my best to help you.
This does not seem to be due to blood disorder.
It can be from a trauma, bite, rubbing.
It would subside gradually.
Please do not get scared.
If it enlarges or if he develops any pain, fever or any other symptom, please get it examined by doctor.
I hope this information will be helpful.
If you have any query, please feel free to ask.
If you are happy with my answer please click on the Excellent/Good rating.
Wish his healthy life.