Enroute controller described icing event when a small aircraft was provided assistance; the pilot indicating a loss of airspeed and altitude.

Narrative

I sat down on the radar position; relieving the radar and radar-associate position. There was a large area of moderate to heavy with a little bit of extreme precipitation in the area. Therefore; with the outside temperatures; I knew that icing could be a problem but I had received no reports of icing. Aircraft X was on frequency flying at 8000 enroute to ABC. I advised Aircraft X about the large area of moderate/heavy/extreme precipitation along his route of flight. Aircraft X asked me what altitudes the precipitation was at. I advised Aircraft X that I only showed this precipitation FL240 and below. I told him that I did not have a stratification lower than this. Aircraft X asked to deviate to the east. I cleared Aircraft X to deviate east when able direct ABC; maintain 8000. Aircraft X accepted this clearance and started deviating eastward. His route of flight kept taking him further east; away from the depicted precipitation. I rerouted his flight plan so it would process into the proper Approach Control. I was working several other aircraft also deviating and departures off of the terminal airport; when Aircraft X called on saying that he had ice on his wheels and wanted to return to ZZZ. I vectored Aircraft X towards ZZZ and descended him to 4000. Aircraft X took the clearance and started descending. I asked Aircraft X what kind of ice (light rime) and the outside air temp (minus 3). I submitted the PIREP. I called in the VERSI time to ZZZ TWR for Aircraft X. When Aircraft X was around 7000; I asked him if he still had the ice. He said that the ice was off his wheels. I was then still vectoring Aircraft X for ILS3 at ZZZ. I was working other aircraft and even a departure off of ZZZ. Aircraft X called on about 20nm south of ZZZ saying that he was losing airspeed and altitude. I advised him that the MEA was 2900. (The supervisor at this time got me a D-Side.) I read him the weather at ZZZ. ZZZ was the closest airport for Aircraft X. I called ZZZ TWR and asked him for the information around the airport. Tower said that the airport was clear to the northwest. Aircraft X was descending below 2900 and climbing back up. He was flying westbound when had cleared him northbound to ZZZ. At one point Aircraft X was down to 2100 and then I saw his Mode C read 3200. I advised Aircraft X that the closest aircraft to him was an aircraft at FL200. I told Aircraft X to do what he needed to do; meaning that if he needed to climb that it would be approved. I should have then reiterated the MEA was 2900. Aircraft X finally was able to fly to the airport. I gave him the weather again and told him that tower said it was clear to the northwest. Tower approved for me to keep Aircraft X coming in. Aircraft X finally got the airport in sight and I cleared him for Visual Approach to RWY 3 at ZZZ. I switched him to Tower and Tower called when he landed safely. Recommendation; when I told Aircraft X to do what he needed to do; I should have told him that he may climb to any altitude he needs. I could have assigned him a block altitude. What I needed to make sure he understood was that at all times the minimum safe altitude was 2900. I should have asked for a D-Side when Aircraft X was picking up ice because it took me some time to enter the PIREP.