Check Gun Broker and Guns America. Remington made a .28 ga 1100 field gun with a straight stock. I have a .20ga in it. Very nice shooting gun. I think its called the upland model. I found mine at Gander Mountain in Houston. If you think a double would be ok CZ makes a very nice .28 in the Bobwhite model. I have a .20 ga in that one as well. If you don't mind spending a little more for a nice .28 Ga double look at the Connecticut Arms RBL in .28 ga. That is the one I'm going to get if I get a .28 ga. Collectors Firearms off of Richmond Ave in Houston usually has a good selection and might have what you are looking for in stock. If not they can get it for you usually. I've bought quite a few guns from them throught the years also.
www.connecticutshotgun.com
www.collectorsfirearms.com
As far as shells I'd just get him a case of Bismuth to shoot through it at ducks. Much more effective for that smaller gauge shotgun.