I have a lock mounted high on our front door and the door to the garage. We don't on our deck door bc we have a fenced in backyard. We have combination bicycle locks on our two gates so he can't get out. Here is the link to the type of lock we have mounted high on our doors. You can only use it while home because it prevents people from opening the door from the other side.
https://www.amazon.com/Security-Childproof-Rein...(Phone number can only be seen by the question and answer creators)&sprefix=door+lo%2Caps%2C922&sr=8-3

Can he undo the locks? My son can occasionally but not always turn a deadbolt. However the doorknob covers they have he has yet to be able to undo. There are sticks, chains, alarm mats that go off when people step towards the door. This is difficult and honestly has cost me a great deal of sleep as you cannot supervise someone every second alone because at some point you will need the bathroom and sleep. My son won't really tolerate wearing anything but if yours will Angelsense may be a good invest. Not everywhere but some police stations programs where they collect info voluntarily given about autistic individuals they may interact with. I make it a point to tell anyone I see who is working with my child teacher, school nurse, IEP attendees, bus drivers, speech therapist to be alert that there may be elopement attempts.

You can also get alarms that sound when a door has been breeched.

Get locks on the door that need a key to open.

Maybe in the mean time... try to show him pictures that explains what can happen if he runs out without mommy.
I pray little guy gets all the resources and support he needs.