Guinea Pig Don't Dream of Outside the Prison.

In the near future, research on male pregnancy is being conducted to combat population decline. Nagisa Fuyumi, forced into becoming a test subject despite not wanting to get pregnant, has lived at the institute for five years without ever going into heat. Trapped in the worst situation of his life, he unexpectedly reunites with his childhood friend, Hayato Hinoｓaki—a man he once admired but grew disillusioned with. Now, Hayato is both a test subject and a researcher at the institute. Their fates intertwine once again, but this time under very different circumstances...

(Source: J-publishing, translated)

Note: Includes one extra chapter.