Why is my baby speaking only in vowels?

How old is he? Babies start vocalizing with vowel sounds. By 9 months, you should start to hear consonants such as ba, ma, da. If he is older than 9 months and only uses vowels, you should ask his pediatrician about a speech therapy evaluation.
Cooing... ...Is easier than babbling. Consonants require more complicated lip and tongue and breath coordination.