How to ask my bestfriend to masturbate with me?

me and my are not gay and were 15 and we watch together he masturbates and i do to i never thought about it until we were watching one time after awhile i got hard and put my hand in my pants and started playing with my his hand was in his pants i told him i could'nt hold it no more he said go in the bathroom thats where he does it he said i said cant do it sittin down so he set me up in his room he gave me some lotion and an old shirt he doesnt use to *** on and then he left me to it he wasnt surprised cause he does it to. Another time was at my house i was lookin at on my computer and he was on his computer i was layin with my hand in my pants jerkin i told him to look at this and leaned over my leg while i was still beatin