I know weight watchers has plans for every walk of life that you can try. Of course by now you are probably finished with breastfeeding your son.
The main reason why we are not allowed to diet is because of the toxins in fat which are released into our system and hence into the breast milk if we lose weight too quickly, and the other is that we may not make enough milk for the baby if we take up too strict a diet.
Eating healthy is a good thing as always. The best diet is not to eat less, but to change what we eat to lose weight....low fat and low sugar and staying away from processed foods will help you and the baby feel healthy and will provide you with some weight loss benefits as well. You can also try the Pritikin or the MacDougal Diet, which restrict food choices, but not calories.