Get a door alarm. Most are in two pieces, going on the door frame, then on the door itself. There is a magnetic field between the two pieces, and when it is broken a 160 db (decibel--about the range of a jet taking off) sound alarms, and trust me, it works. I have two autistic angels and this works for us...
Also, get an ID bracelet for him with your numbers, home, mobile, work. There is also an electronic monitoring system called Project Lifesaver that works like a tracking system. I am not overly familiar with that, but if he does wander, it could be an added safety device to locate your son quickly.
For my two kids, I just have the Medical ID bracelet on each of them. Their names, my and my husband's name and phone numbers, and the fact they have autism and that they may or may not answer questions and try to bolt. All this including rush shipping cost about 50 to 60 dollars. It is worth that and more to know that in the event that they could not answer for themselves, they have a way to communicate this important information.
Good luck.