Help! My crate trained 14 week old puppy barks for 4-5 hours at night

Hi everyone! I’m losing my mind, so I’m hoping you all might be able to help.

I have a 14 week old mini poodle who currently weighs 5lbs. During the day, we follow a 2-1 nap schedule, and he naps in his crate just fine without making a sound. Sometimes, he’ll even take 4-5 hour naps in his crate during the day, and we have even had to start waking him up.

Recently, however, he’ll go down for bed time at 10:30 and sleep just fine till 1am. After his potty break, however, he loses his mind and will bark non stop for 3-5 hours straight. I sleep on the couch next to him, and I have even tried sitting next to the crate with my hand in it, but it doesn’t seem to slow him down at all. I have started moving him to his play pen after the potty break, and he still barks there, which is very rare during the day.

Yesterday, we thought we would tire him out a little more to see if that helped, so we took him with us all day in the sling. He napped maybe two hours all day, but the pattern was the same. The second he got home he went to bed no problem. Then after 1 am, crazy barking till 4 am.

The craziest thing is that during the day he’s not really a barker. He only ever barks when he is in his crate and needs to go. We live in an apartment, and so far everyone has been really gracious, but having him continue to bark for 4 hours every night is not an option.

He also sleeps with classical music on, and he have tried leaving a light on, but none of that seems to help. Any ideas?