1969 valiant with 13 inch wheels

Yes, disc brakes need different wheels than the 13" ones you have now. If you're keeping the small bolt pattern, you're rather limited on stock wheel selection. The ones that for sure work, are the 14X5 1/2" wheels. Some of the 14X4 1/2" wheels work too, but, it depends on the year, and which brake kit you buy. With aftermarket wheels, the sky's the limit, depending on your budget. If you want to use some stock dog dish hub caps, and not the stock 14's, you may have to do a bit of looking to find some with the correct diameter nub pattern. I think some of the vintage wheel companies will make them for you. I would recommend that any aftermarket wheels you get (whether off the shelf or custom made) you get 15" or bigger ones for better tire selection and cheaper prices.
i just know he wants to keep it looking as original as possible. My wife and I are the ones opting for disc brakes. he is an old school type of kid.