First of all, congratulations on the new addition to your family, and on the 20 pounds you have lost so far! 20 pounds is quite a bit, so that's nothing to scoff at!
I think the first thing you should focus on is getting your diet in check. It doesn't have to be anything extreme, but it would help to know how many calories you are consuming. I like to use a notebook to track my meals and workouts, but I also use
www.myfitnesspal.com for the same purposes. I think checking out the site might be a good place for you to start! You input what you eat and what exercise you do and it tells you exactly how many calories you are eating with the options to track protein, carbs, fat, fiber, sugar, etc.
Regardless, tracking meals and exercise in some way is probably a good jumping off point.
Secondly, it might be a good idea to come up with a plan of what sort of exercise you can reasonably do each day (since you are a busy stay-at-home Mommy now). Do you have someone who can babysit for you, or a time when your husband can watch your son? If so, that would be a great time to get out and go to the gym again.
If not, how about getting some DVDs you can use at home? Any of the Beachbody programs or Zumba are effective if you couple them with a calorie deficit, and I've heard good things about Jillian Michaels 30 day shred program.
You can also get a running/walking stroller and take your son with you, or get a few sets of light dummbells, bands, etc. and workout while he naps.
There really is NO LIMIT to what you can do if you just set your mind to it.
How does your husband fare in terms of working out and eating well? Can you count on him to help motivate you while you get accustomed to a program? Internal motivation is much more reliable long term than external motivation, but it can help to recruit a buddy to help you get on a program.
The best advice I can give you overall is to pick a couple of things you can commit to changing now, and stick with them. Over time, add in more strenuous exercise, tweak your diet as needed, and experiment. Each day you commit to eating a healthy diet and moving your body brings you a step closer to your goal.
You will have set backs; there will be days that you eat more than you planned or don't workout like you would have wanted to... and that's okay. Its a journey, and most importantly if you can show your son that a healthy lifestyle can be balanced with days of indulgences (Holidays, special occasions, family outings) you are setting him up with a good example of how to live a satisfying life -- and providing him with a healthy mommy who is fit enough to keep up with him.
Best luck!