S.C.
I'm a mom with 4 sons and I really took my cue from them. My youngest is now 9 and he has been bathing on his own since he was about 6 - he has showers which I find my easier. I have one rule and that is that the bathroom door must stay unlocked while in the bath or shower. I frequently talk to him at the door too, so I know that he is ok.
Also you my have to do an inspection of cleanliness, as boys often forget to use soap and shampoo or to clean whole legs. I have been known to send him back. lol
Hope this helps.