Your question may sound strange to many of our Readers, but you are right. Men can also lactate, but it is not as easy and simple as stimulating or sucking the male nipples as you think.
Both women and men have breast tissue which contains hollow cavities called alveoli, which are lined with milk-secreting cells. But the ingredient for lactation is prolactin, a hormone that's produced by the pituitary gland and stimulates the mammary glands to produce milk.
So the difference is a matter of hormonal influence, first during puberty and then again after pregnancy. Men do have the necessary equipment, but usually they don’t naturally produce the necessary levels of hormones to use it. Provide the right hormones, though, and male lactation is absolutely possible.
Normally, non-pregnant women can have almost double the amount of prolactin circulating in their body than men. This concentration can increase nearly tenfold during and after pregnancy.
Research has shown that male prolactin levels can also increase. But this will require injection of prolactin and estrogen into the male body. I have read that one way to induce lactation from men is via starvation. During World War II, thousands of men reported lactation while held as prisoners either in Japanese POW camps or Nazi concentration camps. When a person is malnourished, the hormone-producing glands—like the pituitary gland, which generates prolactin—are impeded, but so too is the hormone-destroying liver. So, some combination of manual nipple stimulation and hormone injections may develop the expectant father’s latent potential to make milk.
But why do you want to do all this and mess with nature. Taking of drugs is a life long experience and you will have difficulty in finding a wife later on. Stick with wearing female attire. No harm will be caused by that.