Light aircraft pilot reported an NMAC with another light aircraft in A90 airspace.

Narrative

My student and I took off from OWD at XA56L direct to ZZZ on a training flight and would proceed to level at 2;500MSL on our way. The OWD tower controller cleared Aircraft Y to take off behind us enroute to ACK. We were told by the tower to switch over to Boston approach for flight following as we had a squawk code and had requested flight following from OWD ground prior to taxiing. Before we switched off of the tower frequency; I heard them give two traffic reports to Aircraft Y behind us. One report to alert them of the traffic ahead and the second to give an exact 'clock' direction; twelve o'clock. Aircraft Y responded that they had the traffic in sight. We switched over to Boston approach and after our initial call we were immediately alerted to traffic 'behind same altitude' by approach. I looked at the ADS-B traffic on my Ipad and then over my right shoulder to see the traffic within 200 feet laterally; same altitude. I have never seen traffic that close in my time flying without it being intentional formation flying. I could read the tail number off his aircraft and was able to easily grab a photo before he accelerated past. I contacted Boston approach and alerted them to the fact that we just had a near midair within 200 feet of separation and when approach got a hold of Aircraft Y (who was getting flight following to ACK) they denied they were ever that close and that 'they had the traffic in sight.'I very strongly believe this pilot displayed a gross disregard for any level of safety and intentionally brought his aircraft within close proximity of a training operation. I do not know what his reasoning was but I cannot think of any good reason to overtake traffic at a distance so close that would endanger lives.After the incident I received a phone number from approach to speak with Boston center. I promptly made contact with them after my flight completed and they informed me that the local FSDO might reach out.