MY Friend is an idiot?

like i know hes tryin to be funny and stuff but he doesnt think when he talks and when he says stuff like (a Baby voice, a bad one) "ADOOPDUGDOO" idk i cant really spell it but thats a gist of what he says, its really anoying and im a complete A** hole to him when he does it because i have had it with the stupid crap he says. Hes 17 and still actin retarted. What should i do when he does this or should i tell him to grow up? Yea and hes a compulsive liar and over exaggerates things.