30 minute naps

My son was napping for an hour in the morning, 2 hours in the afternoon and 2 30 minute cat naps in the evening but at 4 months he started sleeping 30 minutes at a time. He is now 5 months and still only sleeping 30 minutes. He wakes up sometimes happy but most of the time upset. I try to get him back to sleep but he fights me. Anyone dealing with this or have dealt with this? How do I get him to nap longer?