Girlfriends Son

When I first moved in , it was life as usual around the house. After some time he began to wear only his underwear around the house, which was no big deal. His mother had told me he has always done that ever since he was very young. Things started to get different from then on. My girlfriend has a 2 bedroom, one bath house and in the mornings it's impossible for all of us to use it to get ready for work an school. His mother asked me if I'd be comfortable sharing the bathroom with him in the morning. No big deal. If he made it to the shower first then I'd shave and brush my teeth etc. and then I'd get in or vice versa. After awhile , we had a comfort level built up and he would talk to me while he was showering. The shower is all glass so you're totally exposed. When I would shower , he'd do the same, and he would tend to stare at my manhood. I don't know if I'm over reacting, because I don't have kids. At night we eat dinner together and then sit in the family room an watch tv for awhile and then she an I go into her room to get in bed an finish watching tv. I always lay nude in bed and she just wears a tank top. Sometimes he will come in and lay on the covers between us an watch tv. It feels weird being nude right next to him. This is going to sound awful but I have had a few full erections with him laying next to me. I've also gotten hard in the shower with him in the bathroom.My girlfriend knows that I get hard sometimes and doesn't mind, because she knows I'm no threat. She told me there's something erotic about the thought of a mature nude man with a boy. She is open minded , but I didn't expect that. On Thursdays his mom has meetings in the afternoons so I pick him up after school and take him to soccer practice. There is a small locker room at the county field but he doesn't like changing in there so he strips (completely) in my truck to put his soccer clothes on. I just know when I was his age I was more modest about my body. His mom said to me the other day that , if I was comfortable with it, that I could talk to him about his body, because he's at that age where he is getting erections frequently around the house. She has told me I should take him camping/fishing for the weekend and that would be a good opportunity to talk to him.