This sounds totally normal to me! I taught fifth grade for 6 years, and I had to have a ton of talks with my 10 and 11 year old boys about hygiene. The good news: this will change completely, once he starts noticing girls. And that will happen sooner than you think! Until then, be very honest with him. Tell him exactly what you need for him to do, and feel free to bring in a timer when he's starting his shower. Tell him, "You're in here for 5 minutes whether you like it or not. So clean up, kiddo!" (I wouldn't actually sit and watch him though, because privacy is so crucial at this stage for him.)
Also, why don't you have him put his dirty clothes in a hamper in the hallway every night, before bed? That way you can empty it out, and he will have no choice but to wear clean underwear! That seems like an easier battle to win...
PS I totally agree with Nicole. Humor works miracles, as long as nobody gets their feelings hurt!