If your child is going to VVEC, then he has a delay of some sort, correct? Are they trying to mainstream him into a Kindergarten class? What does his IEP say?
My guess would be that he was changed to the afternoon class due to his age and readiness for Kindergarten. But, I have no schooling or expertise in this area.....
What does the teacher say about his challenges? I would not be OK with that - tell me what his challenges are and how the teacher and aides are meeting his challenges. You have a right to expect that your son is making progress in this class. You would not want him to regress, and neither would the staff. You ARE your child's advocate, so you need to speak up if you're not getting cooperation or if you feel like the teacher is not being responsive.
If you check back with the teacher and ask for specifics on what your son's challenges are and still don't get anywhere, talk to the principal. You are a teacher - imagine if one of your previous students was havng a difficult time in your classroom....you'd likely want the parental involvement so that you could help the student succeed. There is no reason to expect less in this case.
Good luck
T.