Missing Cat Returned, But Now Showing Signs of Stress??

My cat went missing for the day which is very unusual for him. He is very good at returning home in time for meals etc. It also has been raining on and off all day and he prefers to stay inside when the weather is bad. When he came back he was absolutely soaking wet (it was pouring down with rain) so I dried him straight away. As I was drying him, I noticed his breathing was very fast. His pupils were very dilated. My dad picked him up and he was very distressed by this (he struggled slightly and miaowed very loudly). We immediately gave him water and food because he had obviously not eaten all day, he didn't want it. He doesn't seem to be injured, his paws, tail and ribs were all checked and they didn't appear to be hurting.

Right now, he is very, very tired (he's currently sleeping) but his breathing is still fast (not as fast as when he first returned, though) and his eyes still wide. He still hasn't eaten. I read up stress symptoms and one person said to check his gums, they are their normal colour. I've currently left him to rest as I don't want to stress him out even more and have left food and water nearby but I have also left food and water where it usually is found.

Is it possible he is a little traumatised from perhaps being locked up somewhere all day? What can I do to help him? 

I've never left a post on a forum before so I'm not sure how this all works! Thank you if you get back to me, any advice is greatly appreciated.

I'd take him to the vet.

Thanks, we'll take him tomorrow if he still seems uncomfortable etc and if he seems to get worse during the night I have an emergency vet number. However, maybe a trip to the vet may stress him out even more? Especially if he's been trapped somewhere all day

Take to vet. Maybe poison, or internal injury. The only way to prevent is to keep inside. It is a dangerous world outside for curious kitties! Hope he is OK

Where you able to, or was it necessary, to get him (I'm guessing his name is Cosmo?) to the vet today? How is he doing?