Quite often when you find these hawk moth catterpillars crawling around, they are getting ready to pupate. Therefore I suggest that you put it in a container half filled with dry multi purpose compost. You will probably find that the caterpillar 'disappears' but after about a week you can look in the compost and find the pupa. Put it back into the compost and put the container in a cool dry place. Eventually, probably in the spring, the moth will emerge.