Really a big
problem for your son, he can contact the dealership and as long as he provides his "prove of ownership" they will order in a new key, or should that now be two new keys that will have blades cut to suit that car's locks. After they have arrived, he will either need to get that car transported to the dealership so that they can connect the car to SEAT mothership for programming the new key(s) and deleting the lost key(s).
Another route could be to get an auto locksmith to pick the locks, remove probably the driver's door barrel to find out the lock code, and for them to cut a key to that code, but how they could then program it to the car remains a mystery to me and maybe others.
There is a lesson to be learned here, never ever, end up with only having a single key for a car, it is a dumb situation to put yourself into - and avoidable.
Edit:- I'd guess that the total cost for two keys, one fully "as was" and one basic working key, will be round about/close to £300.
