good afternoon, my son bought a 10 inch cast iron, and each time after he uses it it just gets covered in rust. what can be done for this.

You have to season the cast iron. Get it really hot and rub oil, reheat then cool. Don't let it sit in water, this rusts it.

It must be seasoned. Google how to season a skillet

Season the pan and it will take care if this.

You can spray/pour cooking oil all over the cooking surface and stick it in the hot oven for a few minutes. Remove and allow to cool, then wipe out excess with paper towels. After I wash mine, I always spray it and wipe off the excess with a paper towel and store it in the oven. (It becomes habit to check the oven and remove the skillet before preheating, so if you store it in the oven you must remember to remove it before turning the oven on to avoid fire/burn hazard)

There's a lot more involved than cooking with cast iron. First you need to season it, which is a process in and of itself. Then...you NEVER wash a cast iron skillet after cooking with it. Which I'm sure is the only problem that your son is creating for himself. If you don't know...ask someone who does.