Thor is on the lean side too. I was kind of bummed by his last vet visit, he's under 4 lbs again.
He was 4.2 lbs last year, 3.8 lbs this year. Which is a 10% loss of body weight, when you think about it. 3.8 is more his usual number though, I think.
Anyway, I was realizing he gets a pretty low-fat diet, with adult dog food and meat treats. Sometimes his hair is kind of dry, so I was wondering if he could go onto puppy food. He does seem to prefer it (Halo). Or maybe it's just the novelty. I picked up a bag of puppy food by accident and I keep that at work, so he gets the puppy food a few times per week.