I don't know where you live but there are programs that work with schools to help children experiencing anxiety get to school. The school social workers and child and youth workers can be a good resource but they really don't have the resources (time) to do a lot of beneficial work. They will like monkey suggested direct you toward local resources.

ROCK (Reach Out Centre for Kids) has locations in Milton, Oakville and Burlington. They have a weekly walk-in clinic where you can meet with a therapist and problem solve. Sometimes one session can be enough to get them over a hump or at least to take a step in the right direction. They are children's mental health centres and specialize in working with children and families. A few of their staff have extensive experience helping kids in managing anxiety.

Other local agencies may have a school support worker or an Intensive Child and Family Therapist that can also start working with you quickly and move your son in a positive direction.