My son fell off a small trampoline and landed knee first he has a bump on his knee an he keeps complaining it hurts. Could it be a bone contusion?

I would. Take them to your pediatrician. If your insurance allows to go to an orthopedic doctor, than i would just go there... Maybe an urgent care center tonight.
Xray. Could be contusion or fracture. Examination by physician and differentiation of contusion from fracture by radiographic exam.