There are tents you can get that fit on a mattress may give him some clearer boundaries...also door alarms on his bedroom door so if he gets up and out you will hear. Triple check to make sure that everything in his room is 110% safe and is not possible to do or cause harm.

My son didn't have normal sleep habits. So I decided to settle that he would just stay in bed all night. There's a good section on the procedure to follow in the book "A work in Progress" by McEachin and Leaf. I would encourage you to take this approach to build "boundaries" rather than using physical boundaries. Or at least, fade those physical boundaries ASAP.

Because my son was a runner and he could open EVERY child safety lock that was sold I bought battery operated window alarms and put them on every window and door in the house. http://www.homedepot.com/p/Doberman-Security-Wi...
Once when he was 3 we caught him trying to climb out of his bedroom window. These were a life saver. I still had to teach him not to do that but at least I wasn't picking up his broken body off the pavement first.

I work for a mobility company but we also supply other products for disabled people. We just brought out a bed to a mom of a few ASD children and it is made by Noah's Bed look them up. It is a solid wood frame with a tent like cover that goes over the top and actually goes under the mattress and secures by looping around all the polls and bottom. It has mess sides like a tent and a zipper again similar to a tent but it is on the outside so the child cannot unzip it.

It is considered a durable medical product so the insurance will pay for it. The mom loves it and she was also happy that when we installed it I knew what she was going through because our children were similar. Retail on the bed is about 4K so it is hard to afford to pay for outright but it is a great product

I have heard of door alarms or wrist bands for children who do this,I would check with your Social Services Department.