Upon realizing it is a sarcophagus, he decides to sacrifice himself (I think by disconnecting his air) and exist in it for eternity along with the original creature. It may have been in an old Omni magazine, or some other periodical. I thought it may have been in the anthology "Dead Astronauts" but I did not find the story in there.
Ben Bova's Voyagers (1981), the first book in a four-book series. An alien ship is detected coming into the Solar System. The only astronaut able to make physical contact with the ship does so as a one-way journey; he will die inside the alien ship.
Spoiler: the astronaut revives because the alien sarcophagus is covered in alien nanotech robots. Nanobots infect the astronaut, bringing him back from the dead. The astronaut is recovered by other astronauts and sent back to Earth. The nanobots become the focus of the next two (three?) books, because the alien nanobots include the dead alien's consciousness, hence the titles The Alien Within (1986) and Star Brothers (1990). The nanobots give regeneration, healing, different powers, but all at a price - the alien judges each person's life up to the point it is infected into, deeming them worthy or not of the nanotech gifts. Several Earth mega-corporations try to strip the alien consciousness from the bots, to no avail. The alien nanobots are trying to educate humans into accepting aliens.