I've been going thru the same thing with my ds. He just turned 3 and always took about a 1 hour nap in the late morning, and I always napped with him. Our new dd was born in november and since then, it was impossible to get him to nap. He would always nurse for about 5 minutes and drop off to sleep and i would lay him in his bed, but after the baby he would nurse for like 30 minutes and he would start to doze off and if i tried to get himm off the boob he would freak out - i mean get hysterical - and wake up in a foul foul mood and not go back to sleep. So I decided f#^% it - I couldn;t sleep with him anymore anyway because dd would always wake up as soon as he nodded off.
He had a really hard time at first, but wouldn't fall asleep unless he nursed or was in the car, and a few times he fell asleep at 4pm, and was up till 9 or 10, but he finally seems to have adjusted and goes to sleep right away at around 7:30 till about 6:00 a.m. When he napped he would sleep from about 8:30 till 5 or 5:30 and nap for about 1 hour. He still wakes up a few times at night, but less than he used to, and he still gets tired from about 11 till 1:30 , but if he does fall asleep in the car and takes even a 20 minute nap, he will be up till 9 or so. Our pediatrician said he should be getting at least 12 hours of sleep a day, which I think he never has done in his life!
I've talked to a few moms who said the same thing - their kids stopped napping around 3, especially if there's a new baby. I think it takes a while for them to get into the cycle of sleeping long enough at night to compensate for the nap - in our case about 2 1/2 months! I hope this helps - good luck!