we went out before bedtime last night, and he came in and peed on the bedroom floor. I think he's mad at me.
Not really. We tend to anthropomorphize animal behavior.

Sounds like the dog needs a lot more structure and discipline and definitely some boundaries.

You mentioned that he's eating grass for twenty minutes when he's outside. That can only mean that you aren't providing a structured walk. If he's in motion, he can't eat grass. Are you just letting him out in the yard to romp? Dogs love that, and there's nothing wrong with it, but an actual WALK with you in the lead is the best thing to do considering the indoor elimination problem as well as his young age.

Once you get the all-clear from the vet, start taking the dog for daily walks, even if you can only spare fifteen minutes at a time. Walk the dog at your side and do not allow him free access to grassy areas until you are ready. Walking for a few minutes first usually stimulates the bowels. When you finally allow the dog onto the grass, it is a reward. Give a "poop now" command and be patient -- it will take a while for the dog to learn what it means. Maggie knows: "cut a loaf," "drop a log," "poop," "go potty," "shit now." Seriously. I thought it was funny to teach her that.

High praise AS the feces slides from the canine chute, and more when done, with physical contact. I recommend petting UP under the neck, so the head is held high, which makes a dog feel naturally proud.

Scold for indoor defection ONLY if you catch the dog in the act or scant moments after. Otherwise they have no clue what you're upset about, but I'm sure you know that.

Some pups can take up to a full year to housebreak, so be patient.

If you are really desperate to initiate a poop outside, you can look up information on the "matchstick trick." It's not pleasant, and I cannot vouch for its effectiveness, but I've heard wonderful things.

Finally, you may also wish to look up a training method called "Nothing In Life Is Free," commonly abbreviated as NILF. You don't necessarily have to follow every single bit of it, but I've found that employing small segments of several training methods has worked very well for me. However you choose to train a dog, it should be compatible to YOUR lifestyle. You are the human, the dog has to learn to live as you please.

Meanwhile, purchase a hand-held carpet spot-cleaner. Bissell makes a good one for about thirty bucks.

Good luck.