I used to have similar problems with weight gain, but I've never had kids. My doctor told me to gain 10 lbs then come back and see her...Yea, that didn't really work haha, but she DID give me some advice on what to eat and when.
-Make sure to carry around high protein snacks with you when you're out. ALWAYS have a snack or two in your purse when you're out of the house in case you feel a little hungry. Carry nuts, trail mix, power bars, peanut butter on crackers, etc. It's a good habit to be in for when your son gets older
Kids get hungry in a split second so it will be good to get used to carrying around a snack in case you're out together when he's a toddler/preschooler and he gets hungry.
-Try adding olive oil to more things you cook at dinner. It's healthy and full of calories.
-Keep working out! If you're lethargic, it can be hard to realize when you're hungry. Not to mention, working out builds more muscle and bone density if you do weight-bearing exercises.
-Make sure to take your vitamins! Take calcium supplements and Vitamin D as well--if you're struggling with weight then it's important to keep your bones healthy.
Now that you're no longer breast feeding, you'll be burning fewer calories per day, so that really should help.