Congratulations, things are working quite normally. Everyone loses weight and then hits a plateau. As you lose weight, simply walking around requires less energy, so you reach an equilibrium. Although a guide states that a man may need for example 2250 cals per day, someone who weighs 150 Kg may require 3500 cals per day. If he goes on a 2250 cal per day diet, he is effectively expending more calories than he is consuming and will lose weight. However, as he becomes lighter, he needs less to move his body mass around and eventually he stops losing weight. At this point he really needs to eat less and start increasing activity. It gets harder and harder but at least, you have seen the benefits so you should be more motivated. Even if you stay as you are, things are better than they were.