The Spanish Gardener is a 1956 film starring Michael Hordern as the British diplomat sent to a disappointing post after his wife leaves him. He takes his young son, whom he mistakenly views as in delicate health, and the son strikes up a friendship with the titular gardener (Dirk Bogarde). The father becomes jealous. It's based on a 1950 book by A. J. Cronin.
I saw this movie years ago on television and enjoyed it. I enjoyed it just as much this time. It's a delightful character study.
TCM has an overview. Variety has a positive review.