15 week old will not sleep alone

Am I being unrealistic to expect my 15 week old lb to nap alone? We have been working on it for a couple weeks and every time I think he's getting better he regresses and I keep having to rock him back to sleep and continue to hold him or he will wake up and cry.  And then he never gets a proper nap so i end up giving up and just holding him.  He sleeps fine at night and only wakes up once to eat and goes to bed around 8:30 and sleeps until 8 am.  Any tips?