I'm sorry to read about the problems your son has experienced with his car. Ford replaced the Focus ST in 2015 with an all-new model (offered with petrol and diesel engines) so I am not 100% sure which model we are talking about here. But I have never heard of a hole developing in the sump on a Focus ST before. It's possible the sump plug (which lets you drain the oil out) has become loose. This would be my first point of investigation (it's a simple fix).

I would be concerned about the replacement engine. What paperwork and history were provided with the engine? It should have a service book from the donor car that shows its age, mileage and full-service history. If it doesn't have this important paperwork then the engine may be older or higher mileage than the seller advertised. Selling the car would be a good idea, but you will need to disclose the replacement engine and this will sadly impact the price of the car quite heavily if the recon engine doesn't have any known history.