A TRACON Controller reported that an aircraft deviated from the SID and flew below the Minimum Vectoring Altitude.

Narrative

Aircraft X checked in departing ZZZ and gave only his altitude leaving. I asked the pilot to confirm he was on the ZZZZZ DP (Departure Procedure). About 5 miles later the pilot asked me if he should still be on the 121 degree heading. I informed him that he should be on the 121 radial from ZZZ VOR. I asked whether the pilot was on the 121 radial or heading since on my radar it is not immediately obvious. He stated he was on a 121 degree heading. I issued a low altitude alert; activated my EOVM (Emergency Obstruction Video Map) and issued a heading away from higher terrain. When the pilot called the OM for the pilot deviation he said that he tried to ask me several times to clarify what he should do but was blocked by other transmissions. The ZZZZZ DP is a simple procedure yet pilots are incorrectly flying it and several other DPs throughout my facility. I think that increased action from FSDO in education and enforcement could help pilots realize the importance of properly briefing these procedures before takeoff.