> Other than telling him get down in a firm voice how can I break him off this
Is there any reason why you don't want to give him a verbal instruction? Most dogs like the sofa, allowed or not, so it makes sense to train the necessary commands from a very early age. I use 'up' for getting onto something (sofa, bench, chair, back of the car etc - basically anything that requires a step or jump) and I teach 'off' to get down from that place. You can use any low, safe surface for training if you don't want to use the sofa but do arm yourself with lots of treats both as a lure if needs be for the early stages then as a reward. Remember that being on the sofa is special so you are going to have to give nice rewards if you want him to shift!
We never used to let our dogs sleep on the sofa as a regular thing but our youngest managed to win us over. However, I still make sure they only get on when there is a suitable cover in place and I have certain bits of furniture they are NEVER allowed on. They seem to learn the rules pretty quickly but you do need to teach them how to get off safely (if puppies) and never give in once you've asked. I often have to ask mine to budge whilst I straighten the covers so they are all pretty used to getting off then getting back on as instructed :)
If you are going to be very firm about the sofa then it might pay to give your dog a new special surface on which to sleep instead. Try creating a raised bed if you can. If you get it right he'll probably not require much too much additional training!!