Pre K - Help! What would you do

My son just turned 4 in Feb suprise lol, he has been at the same daycare since he was 2 he loves the daycare knows all the staff name they know him he’s super comfortable there we visit his old teachers there often, I had no plans on him leaving the facility until he was starting kindergarten. The issue is the pre k room at his daycare is full and they won’t have any slots until September this year when the other children start going to kindergarten, so he’s been stuck in the 3 year old room since his bday in Feb technically he’s just gotten fully potty trained as of two weeks ago and was finally ready to go to the new class and after I talked to the daycare manager she said she would talk to his teacher to be sure and get back with me and when I finally caught up with her later in the week she tried to tell me my son isn’t technically pre k yet and she wouldn’t be hiring another teacher for that class just for him, the conversation got nasty bc I was obviously upset I plan on asking if there is anyway he could do half days or something up until September in the 4yr class or what options he has there, I hate to move my son but it just doesn’t seem right or fair and it’s stressing me out bc he loves his school, he’s been with his grandparents all week for spring break so I’m just trying to figure things out advice