Sorry to hear Jake's appetite is off a bit.
You have received lots of good info and suggestions already, so I thought I would tell you what I do with Shadoe - she is picky but who knows if she is feeling crappy or just being a picky little princess?
There are some foods that Shadoe just LOVES and also some snacks. I tried forti flora and she really loves that stuff!
So, if she seems off and meatloafing or keeping to herself, I test her. I know what she likes and if they don't get her to eat, then I watch her reaction. I should say that she makes it hard because she sits like a meatloaf frequently.
If she suddenly gobbles up her fave foods, she was just playing me.
If she sniffs but does not eat, I put some forti flora on top.
If she gobbles with that sprinkled on top, she was just playing me for the sprinkles.
If she just licks the sprinkles off or worse, just sniffs then looks away, I try with 1/4 tab of pepcid.
If it settles down her stomach and offers of food are taken, then I think she just had an upset stomach, but I keep an eye on her. If it seems to help her, then I give her no more than a total of 1/2 of the 10mg tab for a couple days, just to calm down whatever has upset her.
If you have an upset stomach or feel a bit queasy, I bet you don't have much of an appetite either.
I always has the pepcid handy as I never know if she is playing me, or maybe some p-itis is going to show up. I watch for her BG to go up too.
If he is just a bit off, and nothing happened suddenly, plus if he is not acting vastly different from normal, do a few tests and gather some info to tell the vet.
It's always best to have the vet check everything when you notice changes that concern you. I would rather spend money for b/w and a checkup to find nothing than be at home worrying.
I hope Jake starts gobbling his foods and you have to run out to the store for more cuz he ate everything!