Rabbits can’t vomit, so the poop you saw him throw up was probably not poop but food that had been lodged somewhere in his esophagus/wind pipe. For some reason it hadn’t reached his stomach (once in the stomach food can only come out the other way). Is he breathing normally? Is he responsive?
Do you hear any weird breathing sounds? Like a faint whistling, a faint crackling? Does he lie with his head rised and his nose pointing up? If you do, he probably has breathed in a bit of food (“aspirated”) and he would need to see a vet, because it can go bad. Swelling of the mucosa can ensue, or he could get aspiration pneumonia.
The undigested cecotropes – how do you mean exactly? That he isn’t reaching to eat them? Or do they look abnormal?
There is the possibility of doing a manouver that is called a bunny Heimlich, but it isn’t a Heimlich like you do in humans. It takes great caution and but its an emergency measure aiming to dislodge any bits of food that might be obstructing his airways.
Here’s some good and useful links, google rabbit Heimlich for more info and pictures before you try this manouver:
http://rabbit.org/the-heimlich-maneuver/
http://www.vgr1.com/choking/