To OP, as I read your post, I was trying to problem solve a little bit. My 2 year old daughter has some sensory processing difficulties for which she receives OT once a week, and I'm a speech therapist by training who works with preschool aged kids. It's certainly true that SPD can affect social skills, but I'm sorry that it was put so bluntly by the teacher!

Here are some ideas - maybe you've tried them already, but it's worth a shot. While I think it's good that the teacher is helping your son get into different areas of the classroom to play, it sounds like maybe he needs a peer buddy. It wouldn't have to be the same kid all the time, although that might help your son develop a friendship, but it sometimes works to pair kids up. They can help with snack together, play together, clean up an area together, etc. Also, are you arriving early at school, before other kids get there? This might really help your son adjust to being in the classroom and also help him figure out where he wants to be. And, it really might help to have a playdate after school or on a nonschool day with a friend from class. Keep this playdate short and don't expect to be able to chat with the other mom while the kids play at the park. Although that might happen, you will feel better if you have a contingency plan. Plan to make playdough (or just play with it!), collect leaves and then glue them onto some paper, bake some cookies, you get the idea. Keep the playdate to an hour or so -- leave them wanting more!!

Hope this helps.