I agree with Beth, he's shy,& he's just afraid of you... maybe he was treated rough by a former girlfriends family and is just afraid... I know "my kids" at school are very quiet around me until they get to know me... I usually yell out "Hey ='insert name' " when I see them around campus-- it amazes them that I even know their name. A lot of them tell me that when they first meet me, they were afraid of me, because of my position in our department, but once they get to know me, I'm like a second Mom to them.
Hopefully, they're not messing around if they're alone in the house before you come home...see, that's the Mom in me to wonder!!!