I can't give much advice on the nervous tummy stuff, but I have a dog with severe food allergies, I have tried 6 foods in the last 3 months and finally found one that works.
If you're concerned that your baby isn't eating enough there are a few questions I asked myself on each food we tried before I made the decision to go to the vet or try a new food (remember this is not medical advice and if you suspect anything is seriously wrong consult a vet)
- What is their energy level (high, usually no need to worry, low, see if something else is wrong too, then act appropriately)
- What color are the gums, are the pink or red (good), or dull and grey (bad, sign of either dehydration or lack of nutrients or feeling sick).
- How fast do the gums regain original color when touched? (fast = not dehydrated, slow= dehydrated
-What does the stool look like/smell like? Eventually you get to know what your dog's poo smells like (same goes for gassiness) I can tell if somethings wrong based on that, soft looking or runny = cause for concern, hard as a rock = cause for concern sometimes, medium = not too worried.
A new food takes 2-3 weeks to gain a good grasp as to whether its good or not. Unless you see immediate illness you are best to wait it out on the new food and see where it goes. While we all worry when our babies do not eat, dogs will not allow themselves to starve. It is possible that he just prefers the old food and isn't giving the new one a chance yet.
If you are concerned about the nutrients your baby is getting when not eating, I would make sure that you have Hi-Cal on site and give that a few times a day. I also like to keep some pediatric electrolytes on hand for when he's not eating. I know that they are designed to rehydrate, but it seems like when my boy doesn't eat, he also doesn't drink and this helps him get his bounce back in his step. Some dogs will lick the up out of a bowl (get the non-flavored stuff), and some you have to give it to by syringe.
These are all just some things I've used to make decisions about what I'm going to do with my boy.
Also how much does he weigh? and what is the exact amount he's eaten so far? While it's not ideal, when Harley was sick (3.5lb) I would count his kibbles out, and I was happy if he ate 20 kibbles per day (almost 1/8 cup) it meant that they were getting something, and as his condition improved we worked back up to the regular amount and the regular weight (4lb) eating 1/2 cup per day.
One thing I do now is have a book where I record everything, when food is given, how much, when it's all eaten, when they go to the bathroom, when they are exercised, if there is an episode of some symptoms from illness.... I record it all. That way I can see the trends and know what is normal and exactly what date and time it occurred.
Not sure if any of this is helpful... but its some of the stuff I've been doing over the past 3 months.
Good luck with your new little guy. I'm sure he'll get settled in well. Very cute too btw.