Why doesn't my dog recognize me or my family at times?

My 5 month old German Shepherd sleeps in my bedroom, my son has came in twice due to nightmares. When he was opening my door, my dog sprung up and barked aggressively as if he were about to attack. Then realized it was my son and started wagging his tail. I went outside to get my dog last night after a potty break, and he growled and charged me, but then stopped and started wagging his tail at about 8 feet in distance. Any ideas as to why he would do this?