He is ONLY 6!
Give him time. Public Schools push too hard too quick for reading readiness and comprehension. I was a public school teacher then a private school teacher.
Naturally, children will start to read around 6 or 7 years old. Comprehension comes with time and patience of the reader.
I would refuse the extra tutoring and just read with your child at bedtime or other times that suit you both. If your child is unwilling to read so much, then trade sentence for sentence with him. After each page ask what happened in the story. At the end of the book ask what the story was about. Remember to use terms like beginning, middle and end of the story, and conflict and resolution to the story. This is what will help his mind to wrap around the words meanings. Build up his comprehension slowly while having bonding time with him mom and not allowing this teacher to make him feel like he NEEDS extra help. He is within normal realms and should not be made to feel like he is behind.
When children are forced into reading too early; they do not gain comprehension of words, these letters and words are just symbols on paper that they don't grasp. They have memorized these symbols and sounds but not the meanings. Vocabulary drops in children who learn to read too early because words are not in context.These children tend to loose interest in reading and fight doing so. In this way they compromise their whole education as much learning happens through reading.
http://news.bbc.co.uk/2/hi/uk_news/education/7107798.stm
p.s. it is absolutely amazing that MOST mamas who responded to this post also had this issue when their child was a 1st grader. It is the public school system and pushing children before they are mentally ready to go forward. Do we force solid foods into our 2 month olds mouth before they have teeth to chew? This is what we are doing to children when we force them into early learning when they are not ready. They don't have the teeth for reading yet. Let them develop properly. K and 1st graders should still be working on social skills and building their vocabulary and learning to do household tasks and learning to respect all people and things. Give them proper time to breath and bond with the world, their peers, and family.
Sorry for the rant. After being in the public schools as a teacher and the private waldorf-inspired school setting where reading is taught later and the children are better for it (in my opinion) I have very strong feelings. I, in no way, want to offend anyone with my opinions on school and academic readiness but am open to questions, concerns and comments.