M.H.
It is really really normal for kids of different sexes to play together. Five year olds usually only succumb to the 'my sex only' approach if there is a large peer group around them keeping an eye on what they are doing. Boys and Girls do tend to prefer gender specific toys some, but, not entirely. And this is also more prevalent when there are other kids around to reinforce the choices.
Even if he does choose different friends or toys throughout his life, it seems like the fact that he has a good friend and enjoys a variety of activities is ok.