Handwriting difficulty with older child

My son is 10 years old and has always struggled with the physical process of writing.  He is blind in his left eye, which I’m sure has something to do with it.  How do I deal with this issue?  He needs to learn to write, but he already feels like he knows how to write.  He can form his letters in a recognizable way, but his formation is not proper, nor efficient.  Should I limit his writing, or have him practice more? If I watch him while he’s writing, and correct him about his letter formation, he gets very frustrated and angry.  Shall I leave well enough alone, or “insist on excellence” from a child who may not be capable of it?  I just don’t want him to get to college, and say, “Gee, Mom, why couldn’t you have taught me to write better?”.