The very handsome Blu does look too thin - his ribs & spine should have a layer of fat on them - but otherwise he looks to be in great athletic condition.
First & foremost, has he always been this lean or is it a relatively recent problem? Has he been vetted in the past month or so, with a full blood panel and tests for parasites & thyroid function? Because he's a Boxer, you may want to check his cardio function too. Parasites, hyperthyroidism, and cardio issues often result in loss of body fat despite proper diet.
If there's no medical reason for the lack of body fat, investigate other possibilities. How old is he? If <2 yrs or so, he may not have completely bodied out & that can account for the leanness. I'd guess this is not the case because, judging from his muscles, he's bodied out just fine. (Boy Howdy, he's a doggie Stud Muffin!!
) Do his genetics indicate he will always have a lean physique? Does he have a high activity level that requires a quantity of food greater than what he's getting? Is he stressed? If he's still intact & there are nearby females in season, he may have a tough time keeping body fat. Maybe he just needs a different food. I think I read that you feed Canidae - it's a premium food, but it may be too "hot" for him & thus elevates his metabolism, which causes him to not develop & store body fat.
Satin Balls are good for weight gain; we've sometimes used them on Dobes that come into rescue in very emaciated condition. (BTW, dogs are indeed susceptible to salmonella. Several have contracted it & died as a result of being fed the BARF diet or having been given uncooked bones w/marrow inside.) A commercially available supplement that dogs love and works well to add weight is "Nupro". I prefer it to Satin Balls because of the safety factor (no raw meat that will spoil) and convenience - it's in powder form & is added to the kibble. It's available online through Omaha Vaccine, Revival, and probably other sites as well.