To see if he is too skinny I would need to see a picture of him from above - specifically his back half but he does look a bit on the slim side from what I see.
We have a picky eater and were supplementing with wet food, Parmesan cheese etc just to get him to eat. If we didn't, he would just look at us with the - really you expect me to eat that?? - look. We have cut all of that extra out and have switched his food to the Costco Natures Domain grain free salmon and sweet potato and he loves it. He doesn't always eat right away and sometimes he has his breakfast for dinner but that is okay in this household.
If you are really concerned about him being skinny, you should always have a work up done by a vet too.
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