I agree with margy...If you don't have to, I would stop sending him. His world's been thrown a little off balance anyway with the new sibling. There really isn't any benefit to him going to preschool. As you said he has friends he plays with outside of preschool so, I would keep him at home with you or even his grandparents if you can.
Perhaps he isnt ready for pr school;.there didnt used to be one, this idea of having a school before a school,he has also had a younger sibling come into the family recently with his 6month old sister ,this would make him feel vunerable,possibly left out, as he also has some roughness to contend with at this school I think it would be better for him to stay home with you, it is better for children to be with their parents as much as they can in the formative years.I appreciate this isnt always possible when parents have to work but if you have the time why make him go somewhere at this young age when he doesnt want to and gets so upset,who does it benefit?not the child.good luck I realise you only want the best for him.