baby doesn't sleep. like ever!

I'm about to lose my mind. My son falls asleep but wakes after 20 minutes. My doc said some kids are just cat nappers, which I'd be okay with if he slept at night. Nope. He does one 4-5 hour stretch and is up every 2 hours after. It's very hard!! He's my third. Everybody says sleep when baby sleeps and I'll try but it's torture to fall into a deep sleep to be woken 10 min later during his nap time. Any advise????