ZAN Controller reported an aircraft encountered icing conditions and descended below ATC published routing which resulted in the aircraft flying below the MIA and a CFIT event.

Narrative

Shortly after achieving level flight after departure from DLG; Aircraft X requested his approach clearance into ZZZ and then requested a higher altitude prior to IAF due to icing. A few minutes later he requested to return to DLG and that he was in too much icing to continue the approach into ZZZ. I cleared him to DLG and issued a 30B80 to allow him to climb out of the icing since the MIA (Minimum IFR Altitude) in that area is 50. He then stated that he was accumulating clear icing as well as a reduction in airspeed and requested lower. I informed him of the MIA and he restated that he needed lower. I cleared him to 40 to keep him at least 1000 ft above the mountainous terrain and allow him to get to an altitude to stop accumulating the clear icing. Eventually he got established on the approach at DLG and was able to land without further incident. DLG FSS reported that the aircraft was experiencing severe clear ice.I could have been more suggestive with the altitude assignment as it was below the MIA but there were no other published routings or procedures available to allow for assignment of a lower altitude.