E.I.
dont worry! a lot of boys take interest in other things because they are learning that there is a difference, and they are just trying things out! dont dont dont worry. its not like he is 5 and deciding to be gay or something. thats just not how it works.
just have patience. if you or anyone else gives him the impression that he "shouldnt" play with certain things, then hes only going to wonder why. he just wants to play, have fun, do things with his friends. theres nothing wrong with him and there also is nothing wrong with doing things without his best friend - its most likely a phase, and though its perfectly normal for boys and girls to be best friends at young ages, its not for any other reason than having fun and playing.
i dont know what else to say. its horrible the preconceived notions that boys have to play a certain way and girls have to play a certain way, nad if there is any variation, then theres something wrong with the child and we need to change them. dont worry! theres nothing wrong with him! just let him be a kid, and dont make it seem like its 'forbidden' or something.
www.askdrsears.com might have some info, otherwise, i know either parenting or parents magazine have gone through this topic. look it up! :D good luck!