prom date timing, when is the right time to ask someone? Options

[cheerbee07]

i searched this topic, and didn't find anything. i apologize if i missed it though.

background info:
i like this guy. he's a sophomore (i'm a junior), but he's less than 10 months younger than me. he's basically been my closest friend for the last few months. (and i've liked him since late september) however, he also likes one of my best friends, but i know for sure that she doesn't like him (she's also in a long distance relationship that she didn't tell him about.) btw, he's also really shy.

i asked him to prom on monday, because everyone has been gearing up for prom around here. (our prom isn't until april 29th though). he basically said that it was too early to decide. i'm kinda unsure whether he really meant that, or if he was just hoping my friend would ask him. since then, he's barely talking to me ( i think he's only said one sentence in my presence since then, and never anything directly to me)

therefore, my question is:
did i commit a faux pas by inviting him to prom now? and if so, when is the proper time frame to ask someone to prom? (if there is such a thing)

i'm curious so that in the future, i might have better timing haha.