I would really encourage you to start letting your 7th gr. go independent with science. He’s reaching a level of specific requirements now and it’s not about what you necessarily like, in terms of switching up, as it relates to him. IMO, he’s got to start developing (after the Tiner series), the ability to stick with one topic for a full semester and/or year as part of preparing for high school, and any type of college-level work.
You may even consider enrolling him into a co-op or an online course for Life Sci and Phys. Sci, if you don’t have any co-ops around (especially for Phys. sci.). Landry (purchase from an individual who previously bought in bulk), BJU (they have $99 sales for online courses), Virtual Homeschool Group (free, with live or at-your-own-pace) there are others. With a co-op class or an online one, you can oversee his progress and help him develop valuable time-management and “this assignment-is-due” skills. Then, you can continue nature-study with the younger ones, switching those topics around (even seasonally) as suggested earlier; but long enough to have some immersion for deep learning and not hop-skipping and possibly interrupting that process.
Doing the Tiner books with the guides now is a good foundation, but I think all of them would only be worth 1/2 credit, right?
Then, he’ll need to complete 1 credit of Life and then 1 credit of Physical Science. That takes him into 9th grade or 10 grade to begin Biology.