Sudden school refusal!

We are into the first month of the first grade and my son has had sudden school refusal and bad behaviors. To preface, we had a challenging early years with tantrums and defiance which we did OT and some behavioral counseling. Everything was so much better after 4 and school was the one thing he excelled in. He’s never had issues with school, begged to go to school, worried if we’re running late. The teachers had nothing but great things to say about him. Well, he had a great start to first grade but then suddenly, he’s refusing to go. He cried all morning and was having a huge meltdown. I couldn’t take him even if I forced him to go because he was extremely dysregulated kicking and screaming. And I also didn’t want to make my daughter late so I had to move on and leave him with my husband. On top of that, his old behaviors have come back. He’s been having lots of daily meltdowns again which he was doing so well maybe only having 1-2 a month if any. I’m at a loss, I ask what it’s about, if anyone is bullying him. He just says he’s bored and he hates it.