Pilot reported a NMAC with a military aircraft while on an IFR flight plan descending for approach.

Narrative

I was en route on an IFR flight plan from ZZZ to ACK. I was talking to Boston Approach on 188.2. ATC initially routed me direct at 3;000 ft; and later cleared me direct to HOKEL (heading ~ 160 degrees) for the RNAV 15 at ACK. Approximately 2nm NE of FMH; I entered IMC in cumulus clouds. I briefly exited and re-entered clouds once or twice. Around XA:44; ATC instructed me to descend to 2;000ft in preparation for the approach. The next events occurred over approximately one minute. During the descent; ATC reported 'Military traffic 12 o'clock; less than 3 miles; orbiting at 3;000ft' I responded 'looking for traffic; but I am in the clouds.' About 20 seconds later ATC stated 'I don't know why they are descending; turn right heading 240' I set the autopilot to turn to 240 and repeated the instructions to ATC. Shortly thereafter; ATC gave another traffic call with more urgency in his voice; 'Traffic 12 o'clock; less than a mile; appears to be descending; indicating 2;200; turn right 240.' Again I repeated 'right 240'. A few seconds later ATC gave new instructions 'Traffic is now turning; turn left 150' and I set my autopilot to 150 and repeated the instruction. Around this time I checked my ACAS (traffic screen) and saw a Traffic Advisory symbol overlaid on the own aircraft symbol showing -3. This indicates that the aircraft was very close and about 300 feet below my aircraft. A few seconds after beginning the turn to 150; we exited the clouds and immediately saw a military airplane. The aircraft was less than 500 feet below us and offset from us by only a 100 feet or so. The aircraft was in a nose-low right bank and passed beneath us and out of view. I reported 'traffic in sight' to ATC and was then returned back on track to HOKEL and completed the rest of the flight uneventfully.