First of all, it's generally better to say "I'm dating someone who has bipolar disorder" then "I am dating a bipolar" or "I am dating a manic." The person more important than the disease. Like, you wouldn't say, "I'm dating a cancer" would you? Just to let you know.
But really, you've only been dating three months, and it isn't that great. Why are you still with him? He's not replying, not calling....
If you really like him and think you should stick it out, maybe you think you're in love or you see some sort of future, then yes these behaviors could be (but aren't necessarily) from his disorder. I'd ask him! It's a fairly young relationship and there's no way to tell if he is acting this way because of his disorder or because, well, he's not all that interested.
If he's in a depressive state, sleeping a lot is common as well as a lack of interest (generally in life) and that could be causing this behavior. You need to talk to him to find out.