Meet Me in the Stars by Maggie Hemlock

After losing his true-mate, a widowed alpha dragon gets a summons to space from a dragon flight who left Earthside long before he was ever born. Torn between the past and the future, he answers the call that changes his life and maybe the history of shifterkind as everyone knows it.
Omega dragon, Elio Starscale, has kept a lot of secrets from his true-mate, but that’s mostly because he hasn’t met him yet. With a bit of magic and a whole bucket of luck he comes face-to-face with the man he’s only met between lives.
When neither is what the other expected, can true-mates make love work and reunite the Starscale Dragon Flight with old allies from Earthside?