I’ve lost Thumper Mischief and Amber over a year and a bit and it wasn’t easy. while Thumper was dying Mischief just kept snuggling him as if he knew he wouldn’t see him anymore and after Thumper died Mischief would sit in the corner all day. Benji didn’t act different when mischief died I did all the grieving. and when Amber died again Benji didn’t seemed affected but Luna went from an in your face bunny to sit in the corner bunny.
I just left Mischief and Luna alone for a day just so they could get use to not having the other rabbit around but kept on checking on them
I would just leave the cleaning until your normal cleaning day
and a stuff toy would be great for Obie as long as he isn’t a chewer or you will be needing to replace the toy a lot 
I would just keep checking up on him and give him a lot more affection and he will bounce back in no time as he will realise that he isn’t alone.
hope this helps some what