It's difficult to tell if your son is hyperactive at his age. Even if he is, most doctors will not prescribe medicine to treat hyperactivity in a 15-month-old.
You sound like you're under a lot of pressure with this very active child and a baby due imminently. I suggest you ask your pediatrician to recommend a child psychologist who can advise you on how manage your 15-month-old. Make sure dad goes with you; with four children you'll need all the help you can get. If you can't afford private psychological help ask your doctor to recommend an agency that charges according to a family's income. Good luck.
Our parenting advice is given as suggestions only. We recommend you also consult your healthcare provider, and urge you to contact them immediately if your question is urgent or about a medical condition.