Well, when I first started learning to speak after getting hearing aids, I had a much easier time hearing vowels then consonants (and that's still very much the case today - lipreading comes in handy for that, if I turn my back, and someone were to say "g, b, d, p" - all I hear is the "eee" - so when I was growing up, my mother had to show me the difference between say "p" and "b" - one being softer than the other -it's a labourious process). Sounds like right now, that's what's happening with your kid. He's picking up vowels but lagging on proper pronounciation of consonants.
so, i wouldn't worry yet till your child is older.