Amanda - oh I feel for you girl. You may not have had him on Satin Balls long enough - it took a full four weeks for me to get nearly 1 lb. on MeMe with most of the weight gain in the last week. I think Katie was feeding Wellness puppy food but that can also cause the runs in some dogs.
I know you've probably tried EVO since that's one of the highest calorie foods out there. What about mixing with the Wellness canned food? It's a really nice high quality food. Another good kibble that people are raving about is Orijen's.
Finally one trick that finally worked for me was to...spoil him where he eats.
I feed MeMe in her crate but if she doesn't finish and I need to her keep weight on - I put her bowl on the chair with me and darn if the girl doesn't finish the whole thing. I hate doing it, but she's got to keep the weight on.
As for the treats, the Natural Balance rolls are nice because they've got that extra calorie wheat flour in them if he doesn't have a wheat allergy (which I'm assuming he doesn't since you did okay on Satin Balls). Of course Bailey's K-9 jerky is wonderful but if he's not eating his meals, I'd withhold treats except as an appetite enhancer on top of his food.
Have you tried adding a little warm water to his kibble (1 tbsp max)? Also, sometimes they aren't hungry early in the morning so a bit later giving them back their food can help too. Oh good luck...I know the struggle well.