If the right hormonal levels are produced in a male body, men can lactate.
Science says all male and female mammals have mammary glands. In women, when the mammary glands mature, they produce milk. A spike in the hormone prolactin leads to lactation. In men, the mammary glands do not really mature to that level, and usually, they don’t naturally produce the level of hormones needed for lactation. But if the right hormones are produced in a male body, artificially or naturally, men can lactate.
Artificially, lactation can be produced through medical intervention and injection of prolactin and estrogen. Some drugs like the antipsychotic Thorazine that was popular in the 20th century is known to make men lactate as a side effect. Naturally, lactation in men is rare but possible. For example, during World War II, thousands of men who were held as prisoners in the Japanese prisoner of war camps or Nazi concentration camps, lactated. This was because of starvation and malnourishment. Due to the starvation, the hormone-producing pituitary gland and the liver that destroys the hormones slow down. Later, when some food is provided, the glands recover much faster than the liver resulting in a spike in the hormonal levels, and ultimately, lactation. Alternatively, a tumor on the pituitary gland can cause lactation as it causes the gland to secrete higher levels of milk-producing hormones.