1. post on main
2. we find out who your co-healers are
No, but seriously - I absolutely agree with Tonydanza about calling him out on his derps. And by "calling him out on his derps" it doesn't have to be done in a cold/uncaring/callous way (unless you're into that).
When I started raiding with my current guild in T11, the RL would simply ask people who died "So what happened to you?" and it wasn't asked in a challenging/frustrated/whatever tone of voice. At first I was scared sh!tless to be not only called out on having potentially done something wrong in front of people but also to basically be asked to tell the group publicly what I'd done wrong.
At first I didn't even really know to go into my combat log to figure out what had happened, lol >.>
I quickly realized it wasn't done in any way to humiliate me into "being better" - it was to help *him* quickly assess what had potentially gone wrong (which wasn't always my own fault anyway, of course) so he could see what could be done to avoid it again, as well as the same thing for me. If you have no idea what exactly killed you, you might be wrong about why you died, and fail to avoid it in the future.
And of course the entire raid was given the same "treatment" - whenever he wasn't sure what had begun a wipe, he simply asked those who fell first to help him figure it out. It's not like he was putting *me* on the spot at all. It's just the way the group worked.
Realizing that I was working with a group who all actively wanted to improve, and who all realized that they could fix their mistakes by examining what had happened on wipes... well, it changed my entire way of playing. I never would have been comfortable saying anything to another raider about their performance before that (whisper, raid chat, or vent) and now I actively call for my co-healer to do what I need him to do because otherwise... lol, we don't see a single PW:B from him in an entire night and stuff like that ;)
So... since you seem very invested in staying with your current group, I advocate the "take an active role in Vent on raid nights" and give him instructions when you don't see him doing what needs doing.
Some things might not be best to call him out on in Vent, if you're not the confrontational type; dispels, for example, I'd discuss with him via whisper. At least at first. If he doesn't improve on assisting with dispels by being "gently confronted" via whispers about how few he's taken care of, you might have to start calling for them in Vent, and since that's an... idk, high-pressure situation (managing to call things out in Vent while you're trying to manage healing & dispels can be hairy) you're likely to sound... uh... pissed-off, and even if you don't it's going to put him on the spot in an already somewhat hairy situation, which may not bring out... useful... reactions from him.