Thank you for the information. I am glad you told me, and I'd like to help.
What the other child did to your son was sexual abuse. Anytime a child is used sexually whether it is through touching or actual intercourse, it is considered abuse.
It is unlikely your son recalls the abuse. At age 3, most children have what is called childhood amnesia. Most people cannot recall their lives before age 4. It is a naturally occurring amnesia, not a repression by your son of the abuse. If your son was affected, you would see more acting out from him. However, if you feel he would benefit from an evaluation, that would help rule out any issues. To find a therapist, ask your doctor for a referral. Or you can search on line at http://therapists.psychologytoday.com/rms/.
What your son is struggling with now most likely has to do with something else rather than the abuse. The bullying at school may have caused him to feel bad about himself and may affect his ability to learn and his confidence. It sounds like he may also have depression. Mood swings and lack of motivation are two very common symptoms of depression.
Therapy would help. Also, if you feel your son is open to self help, he can try to work on his issues at home. Here are some resources to help:
Undoing Depression: What Therapy Doesn't Teach You and Medication Can't Give You by Richard O'Connor
Talking to Depression: Simple Ways To Connect When Someone In Your LifeIs Depressed: Simple Ways To Connect When Someone In Your Life Is Depressed by Claudia J. Strauss and Martha Manning
You can find these books on Amazon.com or your local library may have them for you.
Try the on line resources and see if you feel the information fits what your son is experiencing. And if I can do more to help, please let me know.