My 4 and a half mth old son won't sleep more than 15-30 min at a time.?

What can I do to make him sleep longer during the day? He sleeps all night sometimes 12 hrs at a time but during the day it's no go at all and we end up having no free time during the day for ourselves. Any advice? I heard it could be the sunlight but I've tried putting him in the darkest part of the room (Where his pack and play is) but it still doesn't work and I've heard of baby sleeper masks that go over the eyes. But I don't know about those.