Caron, an experienced homeschool mom, shares her thoughts on the why our kids should learn, and master, English grammar. She was asked to compare A Beka grammar to Easy Grammar by another mom whose son severely disliked A Beka grammar.
I can think of two reasons why a child should master grammar:
Towards these ends, I think I would prefer to have your son use Easy Grammar, which he at least tolerates, rather than A Beka, which he hates. If there was a huge difference in the quality of the content between Easy Grammar and A Beka, that would be a different story, but there isn't. Even if there was, I would still probably drop A Beka (since he hates it so much) and try something else: There is no shortage of grammar courses available to the homeschooler. I've used a mix of A Beka, Rod and Staff, Warriner's, Stewart's, and one year of Easy Grammar for various years for various of my children--and that's not even scratching the surface of the grammar programs that are available.
I might also evaluate your son's writing. If he is not evidencing age-appropriate mastery of grammar in his writing, then whatever you're using for grammar is probably not working, whether or not he likes it. Additionally, you could also give him a standardized test, if you know of one that you trust.
See more ideas in HomeschoolChristian.com's Language Arts section.