You might try a high fat, high protein food if you can find one. Acana makes a few options, of course they are super expensive. Fromm's fish formula isn't too bad, I'm considering switching the boys to that.
Payton is pretty much impossible to put weight on. I was feeling pretty good because he weighed just under 21 pounds at the vet last time, and then this morning he lost a whole half-pound... ughhhhhh. I don't worry too much about it because he's not dangerously skinny or anything, but if he got really sick he could probably get into the danger zone pretty rapidly. It just seems to be how he is. This past weekend we were at the trial and everybody was commenting about how Auggie's brother "looks" bigger than Payton... he carries more weight (for the show ring) and also has a lot more coat. I jokingly said that's the "show" model and mine is the "sport" model. I would have a really hard time putting P in the show ring. =P
P is 16.5"-ish at the shoulder, so probably 20-25 pounds isn't too far off as an estimate assuming he gets about that size. I would be happy with P at 22!