
I have bearded collies and recently lost one of mine .... my other dogs were very sad and did have a noticable change in thier character and routine as expected they were missing my girl who was thier friend.
As sad as we were we took them out and about gave them lots of cuddles and extra attention as dogs can grieve as we do. Our dogs tended to mope about off thier food a little and generally quiet ...If the moping lasts longer than 2 wks then I would seek advice from your vet but in our case they came around and gradually returned to thier normal behaviour ... it was ourselves that is still grieving...
my advice would be to return to your usually feed regime but try to fill his time playing ball ..lots of exercise so he cant mope around ... if he is tired out physically he will be hungry and hopefully will eat his meals.
I would think he is eating grass as his stomach is empty giving him bile ...they eat grass to bring up the bile.
Sorry for your loss.