Cardio - take your son on walks to a park that's a 20-30 min walk away, walking at a moderate pace. There's also walking clubs, and probably walking clubs that are just for Mum's!
You can also do jogging, if you're up to it. You could even walk at faster paces each week till you're ready to do jogging :)
There's also high intensity training you can do at home, just google "HIT workouts" (or search Pinterest !)