Horrible napper

My son is 20 weeks tomorrow and has never really been a very good napper. It has only gotten worse the older he gets. He will nap OK in the morning for 20-45 minutes. From then on I have to fight him for obscene amounts of time. The time I spend trying to get him to nap lasts longer than him actually napping. I try to get him to go down as soon as he starts showing he is tired. He will almost fall asleep then he will wake himself back to wide awake. Except he will be extremely cranky and not want to be put back down. This goes on all day. He sleeps OK at night, definitely not attn, but better than his "naps". I'm just feeling very frustrated.