Super Hero Party and separation anxiety creeping in again

Thomas had a great end of the school year this June.  A complete 180 from last year.  He fully participated in field day, including the potato sack race (was blown away by this!).   We had our IEP meeting with the school and he will be eligible for services again next year.  This will include helping him with transitions to activities including gym, music, computers, etc.  He was evaluated by an OT that the school uses and she determined that he is not eligible for this at school, because his difficulties are not affecting his academic learning.   He had a small super hero birthday party with 5 friends at our house that went amazing.  There were no issues with talking or sensory problems with the games.  We let him practice all the games before everyone came.   He is really coming along amazing except for a separation anxiety issue that is getting worse.

We are starting to have a HUGE separation anxiety problem.  Towards the end of school  I noticed him being more clingy on the playground with me and he started not wanting to go on playdates without me again.  I’ll ask him if he wants to go to a friends’ house and he says not unless you are there.  Now that he is home with me all day, he gets upset if I go out to the car to get something or to the garage.  He comes running out the door screaming for me.  I started telling him where I am going, but it doesn’t help.  If we are outside and I need to use the bathroom, he runs back in with me until I’m done.   It is so draining on me and I find myself yelling at him a lot because it is driving me crazy.  No surprise he has trouble sleeping at night now too.  He wants to know what room I am going to be in after I put him to sleep and if he hears the door open down stairs he bolts down the steps screaming for me.   If my husband and I go out for the night, he gets so upset and will wait up in bed until we get home.
Separation anxiety was a huge issue at the end of the last school year too.  Coincidence or is there something to this?

Other than the severe separation anxiety right now, things are going really well.  He is talking to adults, friends, and  children he doesn’t know.   He also tried a trampoline for the first time the other day when no children were on it and has been enjoying his playdates if I stay with him.  

Here's a picture of him from his super hero bday.