My daughter is now 5 1/2 months and she did the same thing. Would sleep at night and then for naps, she fought it like crazy. After we did our sleep training and got her in the crib at night (she sleeps in there for a good nine hours), I thought it would be smooth sailing but we found that we had to sleep train her for naps too. After a few difficult days, following the same nighttime sleep routine, she now goes down in the crib for naps (she also quite often will nap in my arms if she falls asleep nursing which is fine, too). We did the Ferber method on the advice of our pediatrician (yes, I know it's controversial but it worked and our baby is the happiest child I've ever seen) but we waited until she was 5 months. Your son is very young still and probably hasn't really developed his sleep patterns yet. I often felt, in the early months, that we'd never get it right but as she's grown, she has naturally developed a schedule without my forcing it. The one thing that really helped was having a specific nighttime routine. We set her on the bed and sing to her and give her kisses. Then, we have one last song that is the same every night to "signal" that it's bedtime. We give her kisses and put her in the crib. The consistency has been so beneficial in letting her know that it's time to go to sleep. I even notice, now, that when we sing that last song to her, she gets sleepy.
Good luck. I know it's exhausting but I promise you, it will get better.