This happened many years ago when I was younger.
We were in the process of moving to California. My dad had gone on ahead and my mom and I were staying at a friends of the family for a few days. Daniel was the youngest son that was still at home. I was sleeping in the basement and It was Daniel's job to wake me up in the morning. The first morning I'm lying in bed and I feel cold water on my face. I opened my eyes to see Daniel standing a few feet from my bed in nothing but a pair of white briefs, holding a water pistol and spraying water all over me. He was up the stairs and in his room before I could do anything.
The next morning I woke up early and had to pee. Back then everyone wore white briefs and that's what I usually slept in. After I finished peeing I heard the door the basement open and Daniel slowing walking down the stairs. I waited a bit to make sure he was all the way down the stairs then burst out of the door and tackled him onto the bed both of us wearing nothing but our briefs.
I outweighed him by a a good 15 pounds, but he was quick and bit stronger then I thought. It took awhile for me to pin him, we were both out of breath by the time I ended up sitting on his chest holding his arms by his side. We both had hardons at this time. Mine was obvious, poking straight up through fabric of my briefs with a big pre cum stain forming at the tip. His I could feel straining against his fabric poking my backside.
Daniel asked me If I had 'spermed'. I laughed and said no, a lot more comes out then that. He then asked me if he could see. He said he once saw his older brother sperm and thought it was cool.
I pulled the front of my briefs down and tucked them under my balls. I was still sitting on Daniel's chest and Daniel reached up and touched the now free flowing pre cum and rubbed it between his thumb and forefinger, 'cool' was all he said.
Daniel wanted to know if he could make me sperm. I had never had anyone else jerk me off before and the feel of someone else jerking me off felt so good. I was in heaven. He had a nice slow rhythm going on and within minutes I felt the familiar build up of what I knew was going to be a big load.
'Don't stop,' I told him between gasps. 'Keep going when I sperm.' And sure enough I came. The first rope landed over his head, and the next two landed on his face and neck. By the time I had come down Daniel was covered with my sperm, and had a big grin on his face. 'That was awesome,' he said.
I took some of my sperm and returned the favor.