Could he possibly finish up to where you want him to be in the summer? I have a homeschool friend whose son was a slow worker in math, so they assigned the proper credit on his transcript, gave him his diploma the first week in June as expected, then he just finished the work for that course in July. Since they knew he would be diligent to complete it, they did not have any qualms about assigning the credit for his senior year.
On another note, as a parochial school student in the upper elementary years, I always wondered why we never finished our textbooks, and I thought someday the “education police” were going to suddenly appear and take my teachers into custody for that…..LOL!