15 month old refusing bedtime

Hello I am just looking for some advice ! The last couple of months my little boy has been really good at going to sleep for bed time on his own, we lie him down drowsy and he takes himself off to sleep. However the last 2 weeks he is hysterical when we try and leave the room, and he is taking nearly an hour to go off to sleep with us sitting by the cot. He is still on 2 naps a day, is this a sign that I need to transition him to 1 nap or is it just separation anxiety that will hopefully pass !? Once he is asleep he sleeps until the morning it's just a big struggle getting him to sleep!