Yes, request an EHCNA using IPSEA’s model letter.
Alongside this, request alternative provision using another one of IPSEA’s model letters. This is the responsibility of the LA rather than the school. Provision should begin as soon as it becomes clear 15 days will be missed. The days (which can be made up of part days, e.g. the half days) don’t need to be consecutive or already missed and provision should start by the sixth day of absence. Do you think DS is able to attend at all? Sometimes forcing DC to attend is counterproductive and can lead to further trauma.
Also, request a meeting with the SENCO. The school should be making their best endeavours to meet DS’s SEN. So things like emotional literacy support including Zones of Regulation work, nurture group, Lego therapy, drawing and talking therapy, key worker to meet DS each day, looking at support at lunchtime/does DS need somewhere quieter to eat, placement within the classroom, ear defenders/noise cancelling headphones, proper movement breaks… not all these will be possible in all schools without an EHCP bit the school should certainly be providing support.
If DS struggles with crowds and needs routine/doesn’t cope with change, would it be easier for DS to attend just the mornings rather than afternoons? In most schools, the morning is more structured. If he could go in slightly later than normal via a quieter entrance and leave before lunch.