My mare was really fat, hot and barely ridden when I got her just like your guy.
With her I spent a month or two just lunging her every day over poles and taking her on walks and basically barely riding her.
I discovered that once she lost all that weight, through lunging and a different diet, she was much much calmer to ride AND we have a really great bond now because I've spent so much time asking her to do things she does really well, like lunging.
Another thing I noticed is that once she lost weight (she was about 100-200 pounds overweight) her saddle fit better and now she seems to really enjoy being ridden.
One thing I would caution you about lunging is to watch your horse for signs of agitation. My mare would trot along happily but as soon as I asked her to canter she would start sticking her head way up, basically get freaked out and tune me out completely. So I'd just make her do walk trot transitions for 30 minutes and that seemed to be her kind of thing.