First, if the vomitting continues at all on the Wellness, you definately need to get a full work up done by your vet to determine the cause. Vomitting on its own is generally not a sign of an allergy at all. It can signal an issue with his digestive tract/stomach. Eating grass is also very normal and shouldnt produce vomit. Mine eat grass all the time and never vomit. My lab is part cow, she does some serious grass munching, she loves it
I wouldnt worry about him putting the weight back on if he is eating and keeping it down. He will gain it back. Satin balls are generally for emaciated dogs. Weight gain should be slow and steady, there is no need to try and rush it, you certainly dont want to further irritate his system with a food overload. I have a very thin 10 week old lab puppy whos ribs and pelvic bones are sticking out and I feeding him 3 times a day, nothing extra except some boiled chicken and puppy nutrical. He was 7 pounds 2 weeks ago and he is now almost 10 pounds in spite of having coccidia