You wrote: He likes the Captain Underpants and Diaper Baby books, but I'd really like something with a better message (they pull pranks a lot and use questionable language in these books).
I completely understand your desire for him to move on to more worthy books, but in all honesty, I'd be careful! If he is still enjoying those, and wants to continue reading them, please let him. I teach struggling readers online, and for some, they lost interest when they weren't allowed to choose books for themselves. Others never found a series to love, and that held them back.
Choosing a book on your own is a whole different thing that being told what to read. I include myself in that category. Books I read for someone else's sake or assignment didn't attract me or appeal to me, even if I did go back later, read them, and enjoy them thoroughly. Reading is an "in the minute" thing, and him wanting to read them is what will turn him into a lifelong reader! ;D
So absolutely introduce him to some new books, but don't take the others away. Someone suggested the Horrid Henry series, but I don't think it's for you (though he might love it!) as it involves a very horrible boy indeed! I suspect he'd like it. Also up his alley, as mentioned above: the Stink series, and Diary of a Wimpy Kid series.
Here are a few others that can hook boys, though not necessarily him:
The Secrets of Droon (series)
The Dragon Slayer’s Academy (series)
Geronimo Stilton (series)
Time Warp Trio (series)
Hank the Cowdog (series)
Each of these series has hooked many young readers I know. Be sure the books you introduce him to are only a bit higher in difficulty. Too big a leap can really shut a kid down fast.
Good luck! And do consider just grinning and bearing those ridiculous books. He'll tire of them soon enough, and be hungry to find series or titles that will challenge him further, and many of his choices will be about more admirable topics.
Best of luck! :D