So good to hear from you! I cannot believe your oldest is in WH - oh my! Time flies!!!

This is a really good question! I'll just start by saying remember this is a new skill in PHFHG. This is the first year kiddos begin reading independently for science and for a portion of their history (Independent History Study box). It is important for kiddos to take on both readings, as they are essentially practicing this new skill twice a day then. New skills take real time to learn. They are harder at the start, but as kiddos move through the guide, they gradually become better at them. So, remember this is a new skill to practice all year long!
So, how can you help build this skill? Well, you can go over difficult, often used words prior to the reading. You can point out headings, special words, etc. This is somewhat like prereading together. Often simply taking time to enjoy looking at the pictures and reading the captions/headings/etc. (if there are any) is a great way to get the mind set upon the topic at hand. (I did this with the Emerging Reader's Set with one of my sons, and he did so much better with the reading after 'studying/prereading' it first.)
You can also 'set the stage' by reading aloud the first paragraph or two or the first page. This can be done if kiddos need you to 'get the story going' for them. Often times, this is enough to get them started off on the right foot. They now know the gist of the reading and how to pronounce some of the main words.
Be sure you don't require the child to read it all aloud. Independent reading is to be done silently, in the child's head. Reading aloud requires many skills that interfere with comprehension (i.e. thinking how you sound, trying to use exact pacing/tone/pronunciation, etc.). So, trust the child to read it independently, as long as the level of the reading is within reach.
If the reading is truly just too difficult of a level, Sound Bytes phonics might be worth going through. It can often be finished in about 4-5 months, and it is a great way to fill in any gaps for sounding out words/word attack skills. If a student is having to sound out many words as he is reading (rather than just the newer, lesser known longer words), Sound Bytes may be beneficial to do, and if necessary, PHFHG can be slowed to half-speed (and you can help with reading aloud more of the independent reading) until Sound Bytes has been completed.
This son is his own person; just because older brother has dyslexia, that does not mean this son does. However, if over time this son does show himself to be dyslexic, God will walk with you through that too. I know this is so hard, so I am not saying this last part lightly. I had such difficulty with my first son being born so prematurely (though I was thankful for him making it to 32 weeks, as I'd had a miscarriage and lost our first baby before him). I had to work so hard to get my son to do each 'first' thing - to roll over, to crawl, to talk, etc. Then my last baby was premature too. I was so worried, as he had more difficulties than my first. But you know what? God walked with me through all of that. Right by my side. He carried me. Today, I have 3 healthy sons thriving!
I can do all things through Christ who strengthens me. You can do this - no matter what you find now or ahead. God has especially equipped you for this; your children are so blessed to have you. Enjoy each day! No need to worry for what is ahead. You are already equipped for it. And your children are amazing! Just as they are - hope something here helps!
In Christ,
Julie