Brew going through the same thing with my son. His grades suck, his ACT scores sucked and he has little, if any motivation for post secondary education. I struggle with this as I am very disappointed in him but cannot voice that or it becomes a fight at home.
I know I goof around a lot but I am a pretty brilliant mofo and have always been there to help. By my son's age I was a Cat 1, in college, working part time, etc. I graduated in the top 10% of my business school while racing at a National level. I have always been there to support him but cannot hold his hand any longer, at some point as I have been guided, he needs to stand on his own two feet. When questioned about why his grades fell so badly there are a ton of reasons, including nobody to help him, but ultimately he didn't put the work in.
Brew I feel your pain but eventually they are adults and have to fend for themselves.