first off he needs to be seen by a qualified, bunny savy vet to asses the problem, to check the back teeth and to teach you to do it properly. There are people on RO who do not believe in doing it your self and others who do, it is a preference if you ask me. I had a lop who had Malo and i took him faithfuly every 4-6 weeks to get his teeth trimmed, the vet showed me how to do it, i tried a few times but was too much of a spaz to do it myself. With trimming them there is always the risk of clipping them below the gum line, or cracking the tooth to the root and causing a possible abcess, which is not good. Some vets want to put a rabbit under everytime they do it , but to Me that is too much to keep putting a rabbit under anesthesia, i would only do something like that if it was the only option.
If your rabbits teeth are , or i should say that his jaw most likely isnt lined up right you are going to have to monitor and watch his back teeth, because if he is unable to wear down the teeeth properly , the same is going to go for his back teeth to.
I would defintely make your first priority to get him to a rabbit savy vet then go from there.
There is another option of having the teeth removed, many rabbits (one on here in particular i am thinking of) can live perfectly happy normal lives with no teeth, but that is also a big surgery , and a recovery period so that is also something that would need to be discussed with a rabbit savy vet.
Good luck