My 9 year old son also cries easily. I looked at the book the Highly Sensitive child but I don't think it fits my son. He is high energy and outgoing. Sudden changes in routine and loud noises don't bother him. He just cries if he gets his feelings hurt or if he cannot do something the first time he tries. My son also is high achieving but I homeschool and do not emphasize grades. I'm not really sure what causes it. He did go to a part time school and a teacher tried to help me with it. He did better to avoid getting made fun of. I don't know if you should take him to talk to someone of not. I have hesitated doing that because I don't want to make my son feel bad if he is just overly sensitive. I wish I had an answer. But you and your son are not alone.
There's an excellent book titled "the highly sensitive child" by Elaine N. Aron. It can be purchased on-line, in bookstores or borrowed from your public library system (if you are lucky, your school board might even have a copy). If you google the title of the book, you should be able to find additional information. Hope this helps ....
My son, who is now 21, also cries easily. He did get bullied, but I was a fierce advocate for him. Some boys and men are emotionally-based and turn into good men, with support and udnerstanding. You will have to advocate for your son at school with teachers and principals. Might I also suggest enrolling him in an activity which gives him confidence and a sense of strength? Honor him in his difference and be glad you don't have an aggressive child. My son has learned to use his words to express his feelings and to deflect bullying. if his father is not there or is unhelopful, consider enrolling him in Big brothers for a mentor.