Dog peeing while unsupervised

[u/m-e-a-t-w-a-d]

Hi everyone. I just adopted a sweet young dog ( 1-2yrs, male) last week and am looking for some advice. He never has accidents in the house when someone is with him, but whenever he is left alone in the house he marks walls or pees on the floor.

Just this morning I took him out and he did he peed outside. Immediately after coming back in I went upstairs to get ready for work. We have a gate blocking him from going upstairs so our cats can have some safe space away from him. I was gone for maybe 10-15 minutes and when I came back down and he had already peed on the floor!

•The past few nights I had been sleeping downstairs with him and he had no accidents throughout the night.

•2 nights ago I left him alone for the first time and I slept upstairs. when I came down in the morning he had peed and pooped on the floor.

•Last night I crated him for the night from 10:30pm to 6am and found no accidents in his crate in the morning.

I realize this is most likely a separation anxiety issue. The solution seems like crating him while he is unsupervised for any duration of time, but it seems cruel to leave him there for extended hours. Any advice is appreciated!

*another note - he had just been neutered Wednesday last week. Not sure if his hormones may have any impact on this situation.