What is the ruling of the Sharia on a young man communicating with a young woman via messages and messaging her periodically under the pretext of not being ready for engagement and fear that the young woman might think he does not want her?

There is no harm in a man asking a woman for her consent to a marriage proposal, provided it is within the bounds of Islamic etiquette. It is preferable that this be done through her mahram (a male guardian) or one of his sisters, to avoid temptation. It is not permissible to communicate or correspond with her thereafter, as she is a stranger to him, and this opens the door to fitna (temptation or discord). The jurists have prohibited speaking with a young non-mahram woman as a preventative measure. The engagement should not be delayed. It is better for someone who does not have the means for marriage not to propose until Allah facilitates his affairs, while being careful not to make marriage difficult.