Personally, I feel everyone deserves to know their fate and have some control. Maybe my opinion is a little care-bear, but I think it's the "fair" thing to do.
1. Tell him what's wrong. Help as well as you can. If you see he's not even using Wild Growth, point that out in the "what's wrong" conversation. Having said that, it's his toon, not yours. I'm not saying you need to go read up on how to be a world top 100 resto druid and advise him. You need to tell him why you are concerned about keeping him in the group and IF you can find anything specifically wrong, include that info.
2. Give him a reasonable amount of time to show improvement. What is "reasonable" varies. For this issue, my personal feeling would be probably at least two weeks as long as you see improvement each time. For instance, if you see 10k improvement on week one, then that gives him a shot to get another 10k next week and be at the 20k you said he's short.
3. Be supportive. If he needs to run a bunch of heroics for practice on methods/reforging/tricks whatever that he has read up on and you can go, then go with him. Help him as long as he is putting in the effort to help himself. Not only will this possibly help him, it is team building, loyalty building and gives you more time to assess him and his improvements...or lack thereof.
4. If you need to sit him after giving him a fair shot of knowing why and having a chance to improve himself, tell him what it would take for him to earn his spot back. Give him a goal he can work towards. He may not ever try or may not get there even if he does, but at least that is HIS fault for not taking action and no blame is yours for just abandoning him.
"he doens't need healing, he doesn't need healing, he doesn't nee-WHAOSHIT!wtf was that man!". Please stop leaning on TDR. -Teng