He probably wants it to learn some special moves or something, that only Piplup can learn.
Ahhh......my mistake. Maybe he thinks Piplup is cute or something. Prinplup is pretty ugly, now that I think about it.
learns moves quicker, yes
lvls up quicker, no
all forms of a poke lvl at the same rate its just that since your evolved it feels longer
It'll just be weaker.
its stats won't ever be as high. there are few, very rare exceptions though.