That's a tough one. Many bunnies go through a serious and quite scary mourning process when their mate passes. Some mourn and then bounce back and adapt and live well on their own, while others never get back to their normal selves unless they find a new mate. Going off feed, looking distant, and generally just not wanting to do anything is a concern. One of my previous buns was just lost when my girl passed away. She was his whole world and I almost lost him for it too. He eventually paired himself off with another boy and was back to normal (though the bond wasn't the same as with his girl) until that boy also passed away. Poor guy was just lost after that and I nearly lost him a second time. The only thing that brightened his day was taking him outside, and eventually I let him go to another family who had the perfect girl for him and access to a large yard during the day. In the end, I think the bun will let you know what they need and want. Like others have said, if you think he needs a mate, a shelter would be the best place to look. They can do the bunny-dating thing and he can pick out his own mate which usually results in some pretty awesome bonds forming. For now, I would give him a bit of time to 'deal' but keep a very watchful eye on him. Making sure he is drinking, eating, peeing, and pooping like normal.
Poor lil guy. So sorry to hear about your Oreo.