personally i would keep sending him, it gives him time to adjust to the seperation before school is compulsory. can you speak to the school about how to deal with this better - they woudl have experience of this i am sure. maybe make a time out of hours... when they have time to think and speak about it.
you may also be able to speak to him about it when he isnt stressed, find out what the anxiety over it is about. and talkt o him about what happens at hone when he isnt there - he may feel he is missing out. also it must be hard with your brood but some 1:1 time one day after pick up that he can look fwd to may help.
hugs - so awful for us mums dealing with this stuff.