i would first rule out any learning disability or vision problem with a detailed check.
if he is the youngest compare him with your other boys and their personality.
you always have the option of hsing or alternative school. but in the meantime do everything to make sure its just a readiness issue and nothing else.
have you tried other resources.
for instance when i found my dd having issues with reading (different than ur son's) i signed her up for 20 lessons from www.headsprout.com
to see how she liked them since she was a whole language learner and needed help with phonics. she needed help with thinking in phonics style and it has helped her tremendously.
she plays on spellcity.com and she has improved a lot more.
the thing i learnt with her is she is a perfectionist. she wont do something unless she is certain she can do it well. plus i have found the computer is a great learning tool for her. she had it in her to do it - she just needed the confidence push. she was more open to learning reading with a computer program rather than a book. now that she is reading well she no longer reads on teh computer. she reads books all the time. and is willing to try.
also with my dd i found - it wasnt the time you spent studying that mattered. but more how you spent the time and the fun you had.
she recently got a small whiteboard with coloured pens. she loves that thing and will write and practise as play rather than looking at it as homework. she writes - makes mistakes. i dont point them out unless she asks me if that is how it is spelt. she plays teacher student game where i am the student and i make the mistakes she and her class makes.
also do you volunteer? i volunteer a lot so i know exactly whats going on in class as well as what my dd is doing in school. it could be tough for you if you have two younger ones.