Voyagers III: Star Brothers (Paperback, Special ed.)

Astronaut Keith Stoner, member of a joint U.S.-Soviet mission to capture an alien ship that had entered the solar system, was trapped alone aboard the alien spacecraft. But he didn't die, he lay frozen in deep sleep for fifteen years, kept alive by alien biotechnology.
Now Keith Stoner is something more than just a man. While asleep he was inoculated with virus-sized machines designed to repair his body, expand his brain, and let him live forever. Stoner also carries the knowledge and memories of the being whose body was enshrined in the alien craft. He now knows of many different intelligent races who destroyed themselves, battering themselves to death on the stony problems of war, famine, runaway population, and ill-planned technological breakthroughs.
Keith Stoner fears for mankind, and he will do everything in his power--his extensive, alien-enhanced power--to preserve humanity into the future.