I agree with the first person who responded. Check with your school, they will be able to keep you updated. How old is your son? My son is now a senior in high school and for the past 3 years his school uses something called Power School. Each parent and student has a user name and password which allows access to the site. The site has all his classes and the teachers update their grades all year long. One drawback for us, is that the teachers are only required to update about every 10 days I believe. This can be tough at times if you know your child is struggling in a certain subject. If that is the case, I just email his teacher directly and they let me know. When in grade school, we received progress reports half way through each quarter. This helped, but sometimes we needed info before that. If you have a good relationship with your sons teachers, ask them! They may be willing to keep you posted throughout the quarter, semester and school year.
There should be a website you can log into and with the student ID you'll be able to not only see the grades but upcoming assignments, missed assignments and attendance. This has helped me tremendously on keeping my 7th grader on track. In the county that we live in you can register on this website to get notifications on grades (I'm registered for C or below) and attendance in case he misses a class.
I hope this helps.