It may smoke and stutter a little bit, but it won’t cause a “leak in the block”. That can occur in very cold weather when your antifreeze is too weak, or you blow a head gasket, which may ppear to be a “leak in the block”.
Your son must have done something else to the car or the cold weather and weakantifreeze cracked the block.
Ask the mechanic to fully explain what happened and where the leak is.
P.S. I would never put an oil/gas mixture in a modern car; my 1948 Chevy in college, however, used so much oil that it might as well have had a 2 cylce engine.