How can I get my baby to sleep longer?

my son is three and a half months old. I'm not too worried about him sleeping through the night at this point. He does pretty well with that, He only wakes up once or twice to eat and then goes right back to sleep. But it is his naps that have me concerned. He used to take two or three one to two hour naps every day, but the last several days he has only been sleeping for 15 minutes to an hour. He wake sup still tired and grumpy. I try putting him back down, but he simply will not go to sleep. I think I do pretty well at getting him down initially for his naps, but then he wakes up and he won't go back to sleep, and so by the time the next nap rolls along he is overtired and has an even harder time getting to sleep. I already swaddle him, he won't even sleep at all without being swaddled. Does anyone know of anything that I can do to help him?