Why has he been acting distant?
Sadly, this happens more often than not. You're dating someone. You're going out, you're having a time, you're beginning to picture a future together. And just suddenly he becomes distant.
When the communication (effort) on his part dwindles, it is usually one of three reasons:
Reason number one: There’s something tragic going on in his life, such as a family death. People react differently to the death of a close one (or other tragic event) and many choose to grieve alone. Of course, this should only be temporary. And since, in your case, he is behaving this way sporadically, this is unlikely the case so it’s probably one of the following two reasons.
Reason number two: He has simply lost interest in you. The relationship has run its course, as far he's concerned. (Oftentimes, when it’s this reason, he just doesn’t want to hurt your feelings and hopes that eventually you will get the hint and leave him alone.)