It is unfortunate that you never had a court order for support and visitation. Massachusetts uses a formula to calculate child support.
In 2009 Massachusetts made major revisions to the child support guidelines. The new guidelines cover parents with combined gross income up to $250,000.00. The Guidelines consider several factors, including both parents' income and ability to earn income, the number of children involved, and the cost of health care coverage. Only the court can change or modify the amount of a child support order, but either parent may request a modification of the order if the order is at least three years old regardless of whether there has been a substantial change in circumstances, or if the order is less than three years old and either parent's income substantially changes; the child's custody changes; or significant changes to the family's health insurance coverage occur.
Although the duty of parental support is a common law principle, both federal and Massachusetts statutes address child support. Child support may be awarded in a number of contexts besides a final divorce judgment, including an action for separate support, as part of temporary orders during a divorce case, as part of a guardianship petition, a paternity action, or as part of an abuse prevention order under Mass. Gen. Laws c. 209A. Additionally, child support can be ordered as part of an interstate child support action.
Generally, the parent that is the primary physical custodian of the child would receive support form the other parent. More and more couples are sharing the parenting duties equally, and the new Massachusetts child support guidelines addresses this issue as well.
Child support is paid until a child reaches the age of 18, or children over 18 and still attending high school. A number of other discretionary factors may be considered by the court to award child support for children over the age of 18 that have completed high-school, including the child's academic circumstances (attendance at college and the cost), the child's living situation and resources of both parents, and a child's continued economic dependence on the recipient.
If a child under 18 no longer resides with the parent receiving child support, is married, or has joined the military then child support would normally terminate.
The new Massachusetts child support guidelines worksheet can be found here. http://www.mass.gov/courts/childsupport/worksheet-child-support-guidelines.pdf
As you can see from the guidelines, it is likely that you are NOT responsible for 100% of his needs. Further, had a support and visitation order been entered earlier, it is likely (unless there were reasons that you not a fit and proper parent) that you would have had much more visitation.
Since it may be that you have NO duty to pay child support now that he is 18, you may consider speaking to a family law attorney to see if you can have the case closed. Hopefully, you have canceled checks to show that you have supported your child since his birth. Further, now that he is 18, you can see your son as much as you like. However, since he is now an adult, he can also decide if and when he would like to see you.
I hope you find this information useful and that you PRESS THE GREEN ACCEPT BUTTON so that I can get credit for answering the question.