Pikadanny,
I am sorry for your loss. It's hard to lose a bun especially so young and unexpectedly.
It sounds like he could have choked on something. I recently had one of my baby bunnies choke - she was outside eating kibble and running around - she went back to the kibble and ate some then she started rubbing her face - like cleaning, but then she was also acting like she was trying to throw up - I grabbed her and hit her on her back, let her down and she continued these same movements, I walked up to her and saw green yellow type fluid (probably bile) coming from her nose and mouth so I grabbed her face and pulled like I would with a toddler with a runny nose, it was all wet and gooey, picked her up and hit her back a couple more times, she coughed and then was fine. Scared the crap outta me!
I would agree that, if you can afford to, have an necropsy, see what may have happened.
I have not had rabbits all my life, but I believe in researching. I didn't know a lot when I got my Tigger but researched, read, and whatnot; then I, unfortunately, purchased a VERY SICK bunny a few months later and learned quickly how to be a bunny pro, per my vet. He had to be fed with syringe, given anitbiotics via injection, and oral meds for the the first 8 months I had him. We made trips to US DAVIS and my vet locally each month...it was hard.
Binks, my once sick bunny is the SWEETEST bunny EVER! He is like a fluffy, airplane eared puppy. We have a very special bond as I was his mom (years later after raising baby bunnies I realized he was taken from his mom too early - I was told he was 8 weeks but when I look back he wasn't even 4 weeks I'm sure) so I was his mom; he had to be fed and given water through syringe for about a month because he was so young and sick and we thought we would lose him, and his first love but he was sad he needed someone during the day so we got him a girlfriend - now I am jealous of her...but he is happy and loves us both...
Attached a cute pick of Tigger and Binks - Tigger in the last year has warmed up to Binks and enjoys his company outside in the back yard but will share his cage with anyone