I hope you are feeling better now after your Surgery, maybe your son would like more time with you if you have been parted a lot and he has been at his Granma's ,it does read as though the Teacher is the one to speak to ,her interreaction with him regarding sad faces, could be addressed by you,tell her it makes him unhappy when she does that,does she do it with the other children? and Grans do very often let their Grandkids get away with Behavior Parents wont allow ,set some Bounderies but also ignore some of the minor details or at least let them go.
jdtm had good advice for you. these days kindergarteners are expected to know letters and sounds, which is why it is crucial to either teach them at home or send them to preschool. the expectations are soooo much different from when we were younger when they would go to kindergarten to learn these things. if he just started last week, i think it is a bit early to expect him to know these things by learning them at school alone. meet with the teacher and ask her for different activities you can do to help him adjust and catch up. you also may need to negotiate a different system with the teacher. if the sad faces do nothing but aggitate him, they may not be right for him.
First thing - you need to schedule a talk with your son's teacher. Perhaps he/she can shed some light on the situation.