Reality is Cocker Spaniels will most definitely not starve, if he is eating "something" then try not to worry and let him decide how much he needs to eat.
Puppies grow at different rates, some weeks he will need more food than others, don't be tempted to change his food or add anything he is not used to yet as this will just make him fussy.
It could take weeks before he is in a proper feeding routine, and even then he will have days where he eats and others where he won't. Don't be tempted to leave his food down all the time or he will never become adjusted to routine feeding times.
I have had what I thought were "eating problems" with my english setter puppy and believe me english setters are fussy, she now eats very well indeed, although stopped eating almost completely for a few days when her teeth were giving her trouble. I also had similar problems with my swedish import, the upheaval meant he wouldn't eat for days at a time, now we struggle to stop him eating everything in sight and it's only 12 months ago since he came to live with us but it did take a number of weeks to get him into a routine.
And lastly, as a breeder I know it is impossible to predict the amounts each puppy is eating when they are fed together, so when I create puppy diet sheets for my new owners I never stress on amounts, as long as puppy is eating something then things are fine. Another trap some new owners fall into is thinking their puppy is going to eat exactly the amount stated on the packet! This is nonsense, and nearly always the packet will state a much higher amount than is atually needed by most puppies.
My advice would be start to worry if he is not his "bouncy" self and if he refuses food and water for more than 24-48hrs.
You should also speak to your vet about worming as this can affect appetite. And I also find sometimes my puppies go off their food when they have just been vaccinated, so lots of changes for puppy to get used to.
Oh, and get your garden secure!!!! It won't be long before puppy is starting to explore....and all this should be part of his socialisation and getting used to the big wide world.
Hope this helps.