I am really sorry to hear this.
Having no idea what your freezer looks like, or how big your son is, I don't know what to recommend. There must be some way you can rig up a lock for it.
Or perhaps glue on some sort of velcro fastener at a level your son cannot reach.
You of course will need to be extremely careful that your locking mechanism is such that a person cannot get locked inside.
I am sure someone has thought of an appropriate "gizmo" for something like this, but I cannot recall ever having seen one in any hardware or appliance store.
I've only read about a person having a lock on their freezer in the novel "White Oleander," where the foster mother did this to starve her foster children to keep them in line. It was not done as a safety measure.
ETA: I just thought of something. Perhaps the answer isn't a lock but an alarm. They do make battery-operated alarms for doors that will go off if the door is jostled or opened. I think these cost about $15. Of course, you'd have to hang this or affix this at a level beyond your son's reach. And you yourself would have to remember to shut the alarm off when using the freezer, then turn it back on again when you are finished. But this might help "train" your son not to open the freezer.