Our bodies do change from having a baby. It's a major trauma, and it may never go back to what it was before you had your son.
However, the chances of YOUR body going back at 19 are a lot greater than mine doing the same thing in my mid 30's.
If there were an easy answer, we'd all be there, and there wouldn't be billions of dollars spent each year on weight loss products.
It's not as easy as calories in vs. calories out. It's not as easy as changing from white bread to wheat bread. But, my guess is that if you increase your exercise levels and monitor what you're eating each day, you'll have a better chance of getting back to where you want to be.
If you can afford a trainer, get into a gym and target the specific areas you're not happy with. I'd also recommend something like Weight Watchers online that makes you input all your food each day. You may be getting more calories than you know, and this will help you set and reach a target much more safely than trying different diets.
One thing I'd really encourage against is using diet pills (unless prescribed by a physician).
Good luck!