Books by Dawson and Guare can show you how to approach improving EF skills.
3.5 is not “low gpa”. My middle son is a freshman as well and yes, middle school was easy for him and high school he has to put in effort and turn assignments in and he too has tanked his grades a bit. While scary for me knowing this will be part of his permanent record, he will have college options no matter what.
My oldest does have ADHD, lacking EF skills, social immaturity, and learning disabilities. We had his neuropsych testing repeated at age 17. He found it interesting to truly see how his brain works and how he will learn best. It made him feel less of a failure and more accepting that this was simply his own biologic makeup. He discovered he has to be taught in a certain way to best learn things thus he ended up at a tech school for college. They have small class sizes and his major will require lots of repetition and hands on learning which works great for him.