I have a problem keeping weight on my dane, Merlin. I've found that he gains or maintains better if he gets a can of wet food a day, split in his meals. I also break ranks and will give my fur kids a bit of people food in the evening after the humans' supper, if I know that it will not upset their stomachs. Everything from chicken and beef scraps to cooked vegitables and fish has found it's way into their bowls.
I've tried the "quality" canned, like IAMS, but Mr. Picky only likes Pedigree canned or Alpo, and although the others are nutritionally superior, it only works if you can get it in them, so I feed him what he'll eat. I've raised all my dogs from day one the same way, so I've rarely had any upset tummies, but you may want to approach any change in diet very gradually to avoid loose stools.
Stella