My horse Romeo has not been ridden hard in atleast 3 years. (Very fat/lazy/out of shape when I bought him)
I finally got him down to a fairly normal weight, but I want him to bulk up more and be in better shape.
How would you start him to get into shape?
Why not just start riding him. Just saddle up and head out. You will quickly find out what he can take. Spend time on him and forget all the lunging unless lunging is what you want him to do down the line.
I agree with Riosdad, just ride him. Riding him will be the quickest way to learn what he is capable of. Start with some nice walks, if you can ride him for an hour at a walk without him seeming like he is going to blow a foofoo valve then introduce some trotting. Lots of walking and trotting, especially on hills will get your horse in shape.
When I got a fatty into shape, I started out with walk and trot for half an hour, twice a week. After a couple of weeks, I introduced the canter on large circles and for short spurts. I increased the riding to three days around the same time that I started increasing the lengths of our rides. Now I ride him four days a week for 45-60 minutes. Basically, don't push Romeo further than he can go, build up the workouts gradually, and take your time in cooling him off/warming up.
As others have said, hillwork is great for building up those hind quarter muscles! Back when I had access to hills, I'd try to do hillwork once or twice a week. I would warm up my horse then walk and trot up the hills, keeping her in a working frame the whole time and pushing with her haunches. We never did much cantering or galloping up as trotting seemed to work the best for us.