I sincerely apologize for the delayed response. I hope you have been having experiencing more of your son going in the playroom since this post! What you have been doing sounds wonderful and we are cheering you on every step of the way!
Part of the beauty of the playroom is that your son gets to have large amounts of control when he is in there. He gets to play with what he wants, gets to say no to what he doesn't want, gets to be exclusive and ism when he wants, all his snacks and toys he loves are in there, who wouldn't want a place like that to go where people do what you want to do all the time? So although there is one boundary of the door being locked, it is just a dop in the ocean compared to ultimate control he will have while in there.
It's going to be really important that you, yourself feel like this room is an amazing, fun, magical and useful environment for him. That it's a room full of wonderful opportunity and is the best place in the world to help him with his challenges. If there is any doubt in your mind about this, he will pick up on the beliefs and feelings you have about it and that will affect his experience. Also, making sure that you are not "attached" to his going in. He will feel the slight push that will happen if you are needing that in any way.
Next, explain to him again and again what his time in there will be like, print out a schedule and stick it up in the playroom, tell him for example "I'm so excited that tomorrow wer'e going to be in the playroom from 9 to 11 and we get 2 whole hours to spend together playing!"
It's important that he knows the door will be locked and that's because you love him and want to help him! Don't try to trick him or telling him the door in opening in a minute when you know it won't be for another 2 hours.
We wish you all the best!