I’m a Primary School teacher in Ireland and tbh really troubled hearing about what is happening in your son’s school. Based on the information you have given, my gut would tell me that the school are struggling to cater for your son appropriately. The It would be generally recommended to meet with the school – Principal, Class Teacher, SEN teacher, and possibly SNA if he has access, to discuss a way forward. If this brings no joy you may need to contact the SENO for the area (coordinator for SEN – Special Educational Needs) and see how he/she can help. The reality is some schools are out of their depth when it comes to dealing with some kids with Adhd and may themselves need external help. May I ask if the new principal was previously on staff or his he/she completely new to the school?
It is also important to be aware that in Ireland, no student can be denied education which means that the school cannot legally expel your son unless he has another school to go to.