I too, like you, had not had a "little one" to look after for a great many years..when I first got my boy, I specifically asked the owners "what, along with how to & how much to feed"..the amount they told me was shocking..(its only a teny-tiny bit)..still, not wanting to "disrupt" any sort of schedule, I went & bought the exact things they said to feed...I, like you, anxiously monitored & surveyed what went "missing" from the boys' bowls..I was a bit worried at first, just because I THOUGHT MORE should have been eaten/drank...but, still, the pup was fine.
It was something that I had to just "let it go" from worrying myself about. Trust me, when the pup is hungry or thirsty - they WILL eat/drink...no need to get all "panicky" if, at first, there is not much missing from either of the 2 bowls..could be that the pup is just getting itself settled in...give it a few more days still...if after a few more days go by, then, by all means, this MAY be something to mention to the vet..could be at this point that the pup is still in "shell-shock", & may be missing his mum still & such...turns out that MY pup in particular just as the prior owners stated, does just happen to eat really small amounts of food at a time, so,...now, knowing that, things here are fine, & my fella is quite happy, full, & content!!
Good luck to you!!