Your sons grades will either be in transcript form at the guidance office or with individuals teachers, depending on what grades you wish to view. However, if your son is over 18 you do not have access to his grades as he is no longer a minor and privacy laws prohibit you from getting them without his permission. (Such as the case with college transcripts, etc.)
Hope this helps.
Louise Masin Sattler, NCSP
Nationally Certified School Psychologist
Education.com isn't a school, isn't your son's school. You should call or visit your son's school to find out if they have an on-line system for this. There are several. In my school district, the schools use schoolloop.com. At the beginning of the school year, the kids bring home an instruction sheet telling us, the parents, how to sign up for schoolloop and get an ID that is then tied to our kids.
After we do that, we can sign in to schoolloop and monitor our kids' progress.
Your school might have a similar system, but you'll need to talk to the school directly to learn abut it.