Penis's and hormon testoteron's growth

I have 10 years old boy son which will turn to 11 this coming July, I just notice that he has small size penis or even i only could see a hole only when he is naked (he circumcised when he was 3 days old) and I am so worry and brought him to endocrine doctor in Indonesia and the doctor was measured his penis with the condition he was laying down and according to the doctor his size is abnormal which is 2cm  and according to the doctor for his age supposedly 4cm in the relax condition.He got hormone test which is shown his hormone testosterone less than 2.5 while normal range for boys 7-12 years old : 3- 68 ng/ml. which is he is beyond standard. The doctor suggest to do hormone therapy injection. For your information,my son height is 135cm and weight 43.5kg

My questions are :
1. Based on what the standard of penis size?
2. Is there any side effect for hormone testosterone injection?
3. What is the right doctor to visit for the 2nd opinion as i heard hormone therapy could change all the body system.

Kind attention and your assistance to answer would be highly appreciated.

Worry mom
Jaher...