I avoid shooting weddings, for the most part I find them depressing.

However here are my thoughts:

The minimum requirements to be a professional wedding photographer are as follows:
1. Own or have access to a camera.
2. Have a client willing to pay you money.

Since you were not the one who hired the photographer, and it sounds like your advice wasn't asked, it sounds like you might be out of luck. There is still hope.

I think it is reasonable that they would provide a full size jpg of the file. If you get the jpg's you can still crop, and a few other things and get it the way you want it.

Maybe they have some kind of stipulation in their contract that would require you to pay more for a high res photo. I could see someone doing this to make a few extra dollars.

I know what I would have asked of the photographer if I were hiring him. He may have been reluctant to comply and I would understand my request would probably be at a premium cost. Your son may have negotiated something very basic and inexpensive.

See if your son will let you contact the photographer directly and ask for what you want. If it is a real professional photographer I am sure he / she would help you.....for money.