Sonya,
The ladies are doing a great job of helping you talk through possible options!

I am always amazed by the wealth of knowledge and guidance that the ladies offer on the HOD board!

What a blessing to have so many generous moms willing to give of their time to help one another.
I am busily finishing the final editing of MTMM right now but have a few minutes that I will pop-in to offer my thoughts too.

From what you've shared so far, it appears that there are several different issues coming into play with what you are seeing in your son. One issue is that while he loves to read, it seems that this is the only thing that he desires to do as far as his current school day goes without added encouragement or help from you. It is truly wonderful that he loves to read! However, there is so much more to being well-educated than reading.

This is where the struggle is coming into your day.
If your older son is doing Sonlight, it is possible that your next son in line perceives that your older son is primarily reading for most of his school-day while your next son in line feels like he is being required to "do" so much more writing and other types of work as part of his school-day. I share this because my older son did Sonlight for several years, albeit the younger years of his schooling, but it was true that Sonlight mainly focused on reading at those levels with very little writing and very little required in the way of follow-up work. This may not be as true in Sonlight's upper levels, however Sonlight does focus on reading as the source of almost all learning at many levels.
With this in mind, if your next son in line prefers reading, then it is no surprise why Sonlight looks attractive to him. In looking at your second son's needs, however it is important to consider that a switch to Sonlight would cater to your second son's strengths, but it would also leave his areas of weakness still weak. From what you've shared so far, I perceive his weakest areas to be anything written, grammar, and any follow-ups that may not fit within the realm of reading.
This leads us to the second issue which is in the areas of written work, grammar, following directions, and working independently on tasks outside of reading on anything he may not love. While kiddos often balk in these areas, they are also important areas to have firmly in place prior to high school.

With his age in mind, we want to move him forward in these areas.
In looking at the placements you've shared, while you could bump him up to RTR, Rev2Rev, or MTMM in the reading area, what then happens to the other areas in which he is weak? From what you've graciously shared so far, it would appear he has not exhibited the skills needed to move up to the other guides. I'm concerned that if he does not possess the needed skills in narrating, written narrations, DITHR, harder grammar like Rod and Staff, and following lengthy written directions that he will flounder in the higher level HOD guides.
One thing that is necessary to do at this point is to truly take stock as to how much of CTC he really did on a daily basis and which parts he actually completed. If he perhaps was hit and miss with parts of the guide and didn't really do other parts, it is likely that a jump forward in the guides would result in more skipping of parts he is not prepared to do. From looking at the placements that you've shared, I would say that he fits well in RTR. RTR is a step up from CTC in all ways. It schedules Medieval-History Based Writing Lessons from IEW, which I think is a very needed part of your son's day. He also needs to be doing DITHR Level 6/7/8, or another literature program, as there is much more to literature study than simply reading difficult books. Grammar is another area that he will need a strong dose of before high school. Growing with Grammar covers only the grammar portion and leaves out many other needed language arts skills, so I would look in another direction in that area. He will need a full-fledged English program similar to Rod and Staff.
Doing oral and written narrations and following the skills in the guide is another need prior to high school. This is the skill being built in doing all parts of the guide. If my sons do not do a project well, they redo it. They soon learn to try their best the first time around. When a project does not work out as expected, this too is a learning experience, just as when we miss an ingredient in a recipe!
Along with RTR, I would lean toward either doing the science from Rev2Rev instead, or choosing one of the options for beefing up the science in RTR.
If you did ponder skipping ahead to Rev2Rev, you would want to make sure to require your son to do all that is within Rev2Rev without any skipping, as each part of the guide has its purpose in preparing a child for the next guide in line.
I wouldn't advise going all of the way up to MTMM, as the grammar, writing, narrating, and notebooking alone would be overwhelming. At that point, MTMM would just become a reading program for your son, and he would miss all of the rest that was needed.
Only you can know your child best, and know what will suit him best for the future. Preparing for high school is an important part of 8th grade, and with your son's age in mind, you'd definitely want to focus on his weak areas this year and spend time one-on-one helping him to succeed in these key areas. Areas of strength are already strong, so refocusing on the weak areas and doing what is needed in those would be my recommendation.
Blessings,
Carrie