Hi there, Felix,
I can commiserate with the "will he EVER grow up" worry. Kiba was long, lanky and skinny for SOOO long. Also, his coat STILL isn't full, and he'll be three years old on Saturday! I honestly think that the early neutering affected his coat growth, but I have absolutely no evidence to back that up...
The famous Cavalier stomach... Weird, isn't it, how they will eat ANYthing, if they think it's a treat, but refuse, sometimes for DAYS, to eat their dinner?!
Kiba is indifferent to his evening meal. Mornings, well, that's something else all together. He and Paul have a ritual. Paul mixes 1/4 cup of tinned tripe with the contents of a salmon/fish oil capsule and throws in a tiny halibut liver capsule whole. He then lays down the place mat ( a little square of non-skid cabinet liner, so the glass dish won't slide on the floor) with a grand flourish. This is accompanied by Paul, in his very bestest, fake-French accent saying, "Yer placemaat, messewer!!" Kiba is required to sit during this exercise. He is then made to continue to sit for as long as Paul can stand it (usually about 15 seconds, but I do it for much longer!) and then Paul "releases" him to eat his breakfast. All is right in the world for yet another day...
Dinner is whole 'nother issue. I have hand fed Kiba, because I was worried about him. And because it's a nice bonding experience. And because I like my dog. But if I really, really want him to eat, I make it a game. In better weather, I will throw his kibbles out on the grass. In the house, just tossing them on the carpet to make him search for them is fun. Hide them under an ottoman and watch the fun as puppy tries his best to snag those nuggets out from underneath...
His feeding ball is always good for excitement. We add a few treats to his ration and he rolls that sucker around like a maniac till it's dry!! Add another dog (in my case, my daughter's Shih Tzu) and see how fast they eat!!
Seriously, though, if you NEED him to eat, try either a feeding ball, or simply hand feeding him. There are some times when you NEED him to eat, like if you're in a motel while travelling, and you must know when he will poop next.
Always take into account the amount of treats Felix gets in a day when deciding how much to feed him. What you can do is measure out his daily ration, add treats, and just give him that. He won't care if he doesn't get bits off your plates. It's better for him if he doesn't, anyway. I can't convince Paul that Kiba does NOT need him to save two bits of chicken every time he eats, but the boys have their "special" bonding moments, and I would be a witch if I stopped them.
I am very aware of Kiba's weight. I have my hands on him every day, and know if he's gained a pound or two. Since he's been laid up so much in the last four months, he gained almost three pounds! But we cut his portions back, and strictly regulated treats, and now that he's active again, we can loosen up the rules a bit. I like to keep Kiba at between 17 and 18 pounds. As he ages, that number may change, but he looks his best at that weight.
I don't know if this has helped. I kinda rambled off there for a bit!! Sorry about that. If I can help anymore, let me know.