If a son of a so-called or pure Brahmin deploys himself as a farmer with some land then is he called a Shudra? Why not?

It was so in vedic times in the far distant past. A Brahmin’s prime duty was to learn, chant, preserve and propagate Vedas. He could do it only by following the prescribed lifestyle (extremely simple life, light eating, no hard physical work, physical and mental purity, following daily rituals and rites etc).

If a Brahmin by birth opts to work in the field by neglecting his prescribed duties and responsibilities and opts to do ploughing and farming, he is de facto a Shudra.

But not in present times.

In present times, we hardly have pure Brahmins whose life is moulded around Vedas, with total detachment to materialism. Caste has replaced Varna and everything has become hereditary.

For more analysis on the same subject:

Why were Brahmins treated superior in ancient India? Do Brahmins of the present day really think they are superior to other classes of people? Is ‘Brahmin pride’ really justified?