My kids all like playing with everything... 6 yr old girl & 5 & 3 yr old boys. They all play with cars, truck, trains, dolls, dress-up clothes, kitchen stuff, horses, ect. We don't tell them they can't have or play with a toy because it's a "girl's toy" or a "boy's toy". We will be happy no matter what our kids end up growing up like as long as they are happy & good people (meaning respectful of others, kind hearted, lawful & productive in some way).
All my kids do like to walk in on momma taking shower, baths or changing - I don't know the last time I actually got to take a shower or bath without someone coming in and bothering me. And yes I'm asked questions from all of my kids about my breast & if they walk in on daddy - he is asked about his "item" also. We don't remember my oldest (now 15, but doesn't live w/ us) asking about all this stuff, so we laugh & say it's because the younger 3 were nursed, so they have been a little more "exposed" to mommy's breast then my oldes was.
Please let your child play, have fun & enjoy whatever it is that he loves. Kids need to learn from doing everything in life... like I said mine play w/ everything, because someday they will be grown & have families of their own. I hope that they are like their dad & help w/ diaper changes, cooking & cleaning and like their mom & help with painting, home repairs & minor car stuff (mainly just fliud stuff), but we both help eachother out & my kids see this - which is why I think our family don't have issues with the girl's & boy's stuff... cause in the end in a 2 parent family, both parents have to play both roles at points & kids play helps them to get ready for that in the future.