Since the description "content mastery class" is not used in all schools, I will assume that is a part of the special education program -- a class in which the curriculum is modified (presented in a special way) so that this boy can master the concepts and content. If this is the case, then the new school is saying that he is not eligible for such services, since he does not have a real special educational need (even though he has a learning disability). Here's why. A student only has to be provided with special education services if the disability has a significant negative impact on learning in traditionally taught programs. However -- and here's the confusing part -- one sure way that a school system can be in compliance with a 504 plan is to offer special education services! That's probably what happened in the old school.
Now the new school takes the position that even though the boy has a disability, that disability does not prevent him from participating successfully in the mainstream; and even though the old school was willing to give him special education services, he really didn't need those services at all. So they say, in effect: we'll treat this boy fairly and won't discriminate against him because of his disability, but he doesn't really need special education. The real question is DOES HE?
If the parents feel that the special services are necessary (if they constitute an appropriate education for him), then they should ask the special education team to convene to reconsider his status. If it is determined that he has a special educational need, then he should be on an IEP. If there is a finding of no special need (and remember this is not the same as having the diagnostic label of LD), the team may nevertheless determine that he needs accommodations to participate in the mainstream and recommend a 504 plan outlining those accommodations. This is a confusing situation that just might require the services of a special education advocate (someone trained in helping parents obtain special education services in school systems) or a special education attorney.