J.O.
We have a Door Guardian and while it won't keep him from unlocking the deadbolt, it will keep him (or anyone else) from being able to open the door.
http://www.thedoorguardian.com/product.aspx?prodID=DoorG
We bought it at Home Depot and put it up high. The other nice thing about this latch is that if he uses a chair to reach it, the opening mechanism is complex (and strong) enough to keep him from unlocking it.