Toys do not necessarily mean stimulation. Having your rabbit involved in a game is more stimulating than leaving toys on the floor. Perhaps you could build him a cardboard box house, nothing perfect (it will be destroyed anyway), maybe make getting his food into a game. I always keep small boxes around like oatmeal boxes or small food boxes, toilet paper tubes work great too. By distracting their boredom with a puzzle, you can virtually eliminate boredom.
Like I said, my bunny nipped if I was sitting in her way in her old pen and many of the bunnies mentioned above might just feel rewarded for nipping. Spraying them with water might break any fragile trust issues with bunny. I would never risk that. Bunnies can nip if they want attention or if they are simply feeling naughty. It could be pent up hormones if they're not fixed and smell another animal or you. Some like to chew fabric and that's simple enough. When they nip tell them "no" and walk away from them. Circling is another un-ideal behavior similar to the non-aggressive nipping.
Before you act harshly and punish him, try to distract him with a game or puzzle. What worked for me early on was I would pick up Acacia's rattle and ask her to "toss it". She caught on and every time she did I would praise her. It almost wiped out her pen-nipping. I just do not agree with negative re-enforcement. Positive has always been best kind for me and my girls.