Quoting Frankie-marie:" Oh okay thank you, and thank you for the information. I didn't really think about the calories and such that he should be having, def will be looking at it now!"
Not many people think about it- I didn't until I found out Loki had allergies, then I started really studying up on dog foods! Check his package of food on the back and the calorie content on it. Like I said he'll probably need about 2.2k calories at his size.
Also check the first three ingredients. If any of the first three are non-meats or corn, try switching to one with a higher meat content for him to gain weight. If that doesn't work, on my article there is a recipe for "satin balls" which is a huge thing breeders and such use to put weight on their dogs.