Glacier is right!

I am also owned by an Aussie and it is in their nature to be somewhat standoffish towards strangers and like Tamara stated they go through different stages as well. This could very well be either of the two or a combination. LOTS of patience & LOTS of socialization and he should come around. Enrolling him in a reputable puppy class or basic obedience shold help with that as well as other things, like increasing the bond between the 2 of you, keep his listening skills sharp, gives you both something fun to do & is a great learning experience for the both of you.

PS, not to scare you, even though the breed is known to be standoffish towards strangers they are not usually aggressive and it is something that can be easily "trained out of them" with lots of patience, socialization & training.

The NILIF rules are great. They help build a dogs cinfidence, as well as many other things. Are you familiar with NILIF?