If the doctor prescribing the Prednisone was aware of the ulcer, I don't think he would have prescribed it. You can call your pharmacist, or the doctor to find out for sure.
Prednisone can be very rough on the stomach even in someone who does not have an ulcer. However, if the doctor knows of the ulcer, then he/she had decided that the benefits of taking the prednison outweigh the risks of irritating the ulcer. If the doctor is NOT aware of your boyfriend's ulcer, I would definitely suggest contacting the doctor before having your boyfriend start the medication.
In either case, I would also definitely suggest taking the prednisone with food to help lessen the stomach irritation it can cause.