I asked my son's leader to transfer my son, but he was refused.

When your request is declined, keep calm and rational, find other solutions, and don't interfere too much with your son's work, so as not to affect his career development and personal image. You can express it like this:

"Thank you for taking the time to listen to my request. Although I'm sorry to hear that you can't meet my requirements, I fully understand your decision. I also understand that as a leader, you must consider the interests and development of the whole team. I will tell my son the news and respect his decision and your work arrangement. If there are other opportunities or plans, I will contact you again. Thank you again for your patience. "