two ideas:
1) Discuss with his teachers....would they be accepting of him printing everything, instead of writing cursive? (until he gets better at it, anyhow?) Could he use block letters? (show him some examples like architects and engineers use on their drawings) or must he print in the manuscript style that's taught in kindergarden? It's my guess that he'll not be very excited about working from a kindergarden penmanship book....just because it's for "little kids".....and if he can buy into the idea that block letters are a cool way to write to ensure legibility, maybe he'll be more willing to really give it his best shot.
2) Make it a fun challenge. Have him write a full page in his journal, and then show it to an adult who has no idea what it might say. If they can read the whole page without any help from him, he gets a quarter (or more, if you like).
If he has trouble thinking of something to write, have him copy a page out of a book, and then show it to somebody who hasn't seen the book (so they don't know what to expect).
If he can do it in block letters, let him try it in manuscript printing, or cursive. That's worth a little more than a quarter, maybe??
He's 10 years old, after all. He'd probably rather be doing anything than practicing his penmanship. If you can make it a fun challenge though, and help him feel successful when he produces something legible, he'll feel more confident and be willing to try harder, maybe.
Bottome line: He may never have "pretty" handwriting.....but neither do a lot of doctors. The main thing is that people can read it....and that he feel good about himself for his successes and not constantly feel like he's failing at this.
A word of encouragement: My son didn't learn to make letters properly in his early years of school, either. We moved several times, and I think he must have missed that part of the curriculum at each school or something. I worried about it quite a lot....but it was generally legible, so it wasn't a show stopper. I do remember him getting down on the whole school thing for awhile because people made such a big deal about his handwriting. He almost stopped trying for awhile there.
His writing is fine now. He's in grad school. Nobody worries about what his handwriting looks like as long as they can read it. Most everything of consequence is produced on a word processor nowadays, anyway. There are a lot of people with prettier handwriting who never finished college.
Make sure your son understands that the legibility is what matters most....and that pretty handwriting doesn't mean a person is smarter than other people....but of course, it does need to be a way to communicate his ideas...so it needs to be legible.