My experience happened two years ago with my son. I am 45 and my son was 21 then. I'm divorced with my wife for 3 years now, and I do have the custody over our son. My ex-wife happens to be staying with her new husband in Japan, and as for me, I'm dating, but I can't find yet the woman I ought to marry.
It was Saturday afternoon. This is the day that me and my son would go play basketball, since weekend is the only time that we could be together. But this time, we have to cut our game because I have to do loads of work in the office. He understands my situation though. On the way to the office, I've decided to bring all the work in the house and just finish it there. When I arrived at my desk, I packed all the papers and hurried back home.
When I got home, the house was so quiet. I was thinking that my son was sleeping in his room. I don't want to use our desktop computer since I know that my son is keeping some porn pictures and movies from the net so I have to go to my room and do it in my laptop. The first room in the second floor is my sons room. It was closed and I wouldn't want to interrupt him in his sleep. When I passed his door, I saw my door's room half open. Its very unusual because I always leave it closed. I took a peak and I saw that the tv was on, playing my porn videos in a very soft volume. In my bed, there was my son, wanking his lubricated cock. I got a boner upon seeing this. His cock was very huge, much bigger than mine, and the head is so big, it looks like it's going to explode. I was watching him wanking his cock and massaging his balls. I went crazy; I unzip and removed my pants and briefs outside my room. There I am, butt naked outside my room jacking off while watching my son do his thing. I could here his moans and its obvious that he is pleasured with his activity. I was also in pleasure but I couldn't make sounds because he could hear me. It was so intense that my son was rubbing his cock so fast, and I did the same. he let out some loud moans, 'ooooooooh, aghhhhhh, I'll blow, I'll blow baby!!!!!!' and his cum flew out of his slit! It was so strong, that it landed on his chest and on his stomach. My son was breathing heavily as he squeezed his softening cock, milking the rest of his cum. I cant hold it any longer, I moaned and started shooting my load. My son heard my moan and he panickly covered his balls and his dick with his hands as I shot my cum on my door. Some of my cum dropped on the floor and flowed on my hands. I knew that he saw me so I just opened the door. We were looking at each other's nakedness while trying to catch our breath. He then asked me 'why are you watching me?' and I asked 'what are you doing in my room watching my porn?' Then, we just laughed. I went in my room and sat beside my son. We were both naked, but it's fine. I told him its not only basketball that we could spend time together, then we laughed the hell out.
By then, we were watching porn movies together when we're not on a basketball game. We didn't engage in any jack-off session after that, but we would let each other know that we need to wank, and do it privately.