This situation is just awful. I agree with the above posters, do not let your son return to that school and contact your local MP.
I also was under the impression as well that the local catchment school have to take you even if there is not a place, we were actually told this by our local school who were full when we enquired.
I hope you manage to resolve this issue, god knows what a nightmare I would become to the school and the other establishments involved if they were not dealing with it.
Keep us informed and all the best.