Penis problems

So my friends with benefits has this habit of dictating what positions we do and at first I ignored it. The only positions he wants is me on top or doggy style and we can never switch positions so it's literally 1 or the other for less than 10 mins. Earlier today I stopped by his house and really wanted him to be on top this time. He responded with "I don't think that's going to work" but I made him get on top any way. As soon as he tried his penis went soft. So we had to resort to doggy style again. Is this normal? he's only 23. Does it have something to do with me?