My cats gets very disorinated once outside our garden?

First, he is young. He may not have figured out how to jump back over without his housemate's help. Also, there are a lot of sounds, smells, and confusing things outside. Especially outside the safety of the garden and house he knows. Being outside can be scary sometimes, and your cat just feels scared. The rain doesn't help. You should ask your nearby vet for much more detailed information.