TRACON Controller reported an aircraft was below minimum altitude two times during the flight. In both instances; the controller vectored the pilot to the correct altitude but did not inform the pilot they were below minimum altitude.

Narrative

There were two events with this aircraft. The first was a loss with an Minimum Vectoring Altitude (MVA) (Antenna) just west of ZZZ. This was my fault. I was focusing on aircraft in the south area of our airspace and lost focus. I turned the Pilatus 20 degrees left and the pilot reported the antenna in sight and I told him to maintain visual separation with it. The second event was pilot error. The pilot requested vectors for the visual Runway XX and that he had the weather at ZZZ1. I gave him a vector to be around the FAF fix. About 8 miles from the airport I called the field and the pilot advised that there was a low cloud layer and he would need the RNAV approach. I vectored and climbed him out to set up for the RNAV (3400ft at the IAF). I cleared him for the RNAV approach and he immediately descended to 2100ft which is 1200ft below the MVA and he was never established on the final approach course. Low altitude alarm sounded; but I did not relay this to the pilot. The pilot then advised he was having issues with the ILS and would need the RNAV. The ILS is currently out of service. I vectored him back out and re cleared him for the RNAV which he had no issues with and landed safely. I would recommend for myself to issue the low altitude as prescribed by the 7110.65. I should have also immediately asked the pilot if he was established on the approach to keep the situation from escalating like it did. Since the Pilatus is a single pilot aircraft I imagine the pilot was task saturated since he was not planning for an approach and I should have given more time/clarification for him to set up.