Parent Challenges

Being a parent is by and far the most rewarding adventure of my life.  I love it, and I have two really amazing kids, but there are challenges.  My son is 16 and such a wonderful young man to be around, he is smart, funny and respectful.  He is very good to his sister and me.  This age however brings on plenty of stress, challenges  and lessons of letting go.
School has always come easy for him, perhaps too easy. In middle school he was able to do very well, with little effort.  High School has been a challenge, not so much the academic side of it, but the expectation.  Part of me would love to blame the system,but I believe in personal accountability.  Kids are allowed in most classes to turn in papers/homework late, with a slight deduction.  My son has interpreted this to mean its fine, and often turns things in late, or not at all.  Missing the point that the homework is the preparation for the test.  I spent all last year checking the online grading system they have called skyward, then calling his Dad, to discuss and come up with a plan, only to be disappointed with the lack of follow through.  My son made it through with A.B's and C's for his final report cards last year.  This year the first semester started good, then we dipped really low, he lost his Ipod, and the privilege to hang out with friends on weekends if homework was missing (again only at my house), at semester he got average grades, but it is exhausting as his mom to be checking and worrying about this all the time.  I had HUGE hopes for my son, I assumed he would continue to excel through school and go onto a 4 year university.  Now community college is more likely, at least until he puts some effort into school.  This isn't bad, and I am starting to except it, but "I" wanted so much more for him.  Maybe it's MY fear that he will  "settle" like his dad.  I know my son can do better.....
This semester has been the worst yet.  He is facing having to do credit retrieval if he can't pull off a D in Algebra, and this breaks my heart.  I am disappointed in my son for not taking responsibly, and I am mad at my ex for his lack of support on these matters.  We had a clear conversation earlier this semester in which I thought we were both in agreement, our son DID NOT need anymore distractions, and an IPhone would likely be a bad idea, but yet good motivation to work towards if he pulled good grades......what did he get for his birthday, an IPhone.  It's frustrating to say the least!  The various social networks, have become a huge distraction to my son, further contributing to his lack of motivation in school.  I hate it.
The reality of this bad math grade along with his other sub par grades (with the exception of English) has been hard for me.  At what point do I start letting go?  At what point do I stop checking skyward every other day and reminding him what assignments he still needs to turn in?  At what point can I trust it will all work out.....