I can find no mention of a proposed sequel, also, I can't find evidence for the answer given above this does not mean it was once not there.
Here we find a reviewer asking the same question (last updated 08.9.15)
So why have I gone to the trouble of mentioning such idle speculation with you? Well since the first two books were clearly not a duology, am I convinced that Firstborn really is 'the conclusion to 'A Time Odyssey'? Nope. Given this could there be more? Well given that the book ends with a short one-page chapter that presents matters on a new level one cannot but help speculate that there really is more to come. It may be that Arthur had time to jot down some notes and share these with Stephen? If so expect another book. Having said that, and if this really has happened, I really do urge Stephen to take his time. The three books in this series to date do appear to have been hurriedly written and do have some unsatisfactory inconsistencies that could (should) have been ironed out. Of course I can dream... Poole, Bowman and Hal meeting Reid Malenfant. Done right that would really be something.