My daughter has dyslexia, along with vision problems due to strabismus. We were in our 3rd year of HOD (Bigger) and ended up stopping because of frustrations due to reading and math. It wasn't necessary. We met with an educational consultant and she recommended a certain program for math, as well as the Barton Reading System for dyslexia. I could have used those for language and math and kept doing Bible, science and history in HOD.

My dd10 is not reading independently. In fact, she is only to do controlled reading with Barton, and is not really supposed to do writing until we get to a certain level in Barton. She loves to write for fun, so I can't stop that, but I'm concerned about that because her spelling is not good at all. I'm counting on Barton to help in that area as well. She was wearing prism glasses for a few years, but recently was taken out of them to prepare her for surgery to fix her eyes. I can tell that is interfering with her reading also.
For the vision problems, you may want to get your ds checked for eye convergence (tracking problems). My dd's eye problems run deeper than that, but many on this board have said their child had this problem and with vision therapy their reading improved.
As for HOD, if your ds is struggling, I would agree that you may want to stop the extensions. But, I would not stop the science. You read it to him. If he is really struggling, don't require too much or he will hate it. Get those visual problems dealt with and go from there. My dd loves to be read to. I will do that as long as I need to. I am planning to come back in a year and do Preparing with my dd. I may still need to make adjustments for her. I will tell you what I was doing in Bigger ( and wish I would have continued). Map work was out, because my dd can not make sense out of maps at this point. So, for history, I had her do a notebook where we printed off a picture of the person we were reading about and a few lines about that person, and basically did cut and paste. She learns best by pictures, so having a picture helped her to internalize. For science I had a notebook where she would write the Scripture verse stated in the guide and then we would find a picture to go along with the topic. Some she would draw. The experiments we just did orally. I did not have her write them out. I am hoping in a year her reading and writing will be improved greatly, so she can utilize more of the guide. We'll see.
So, don't feel you have to stop HOD, or even cut out certain subjects. As the PP said, adjust it to fit his needs. If you can find videos on a certain topic, great! My consultant said to teach my dd in pictures as much as possible. That is how dyslexics learn best. Look into a special reading program if you are convinced it is dyslexia. Have him checked for eye tracking problems. And, read to him if need be. That will keep him learning the subject as well as increase his vocabulary. I was told to alternate hard(reading), easy, hard(math), easy in my dd's day. Also, I was told to have her do 30 minutes of strenuous activity every day, in which I have not been consistent. So, don't cut out things that your ds enjoys. I hope something here helped and I hope you find something to help with the reading difficulties. I've been there and it is frustrating, because you do want them to love reading. My dd does want to read, but I know it is going to take a while for it to become independent. PM me if you have any questions.
