Rabbits have incisors (what is shown in your picture) and molars or “cheek teeth” in the back of their mouths that cannot be seen without a special tool. Just like in humans, incisors cut food and molars grind food. Both types of teeth can become overgrown. Incisors may overgrow if they are misaligned. In the case of the molars, they can develop spurs that cut into the cheeks and make it hard to eat.
Incisors can generally be trimmed quickly and easily, much like nails. For the molars, anesthesia is usually necessary because they are so far back in the mouth. The price difference between the two types of trims can vary by hundreds of dollars because of this. Frequency varies as well.
When he goes in for his neuter, ask the vet to do a thorough dental exam while he is under anesthesia, if time allows.
Is your other bunny a girl? If so, make sure to keep them separated like MoveDiagonally said. I would actually go one step further and recommend they be separated until she has been spayed as well.