I have a lovely 18 month old neutered male whippet. Since day one when I got him at 8 weeks he’s been fussy. While a Young puppy his weight was fine but since about 8 months he’s definitely been on the lean side. He put on some weight recently and was looking better but I weighed him yesterday and he’s lost weight 
He’s never loved his kibble but used to eat a reasonable amount of it. These days he picks at it. His main meals are good quality tinned and a chicken and pasta loaf for breakfast because he won’t eat the tinned for breakfast. I was cooking his breakfasts for a while with beef mince, rice and eggs and he seemed to put some weight on with that so I’m considering trying that again. I’ve tried putting him back on puppy food (vet suggestion) but it doesn’t help.
To complicate things further he’s a highly strung dog and is currently on medication for separation anxiety.
He doesn’t appear lethargic and loves his walks. He often trots the whole walk rather than walk. When I take him to the beach he happily goes for zoomies. His coat looks good and his eyes are bright.
I find myself constantly worrying about him and feel it’s detracting from my enjoyment of dog ownership.