Either that or he had something really big going on with the mound-builder civilisations that failed spectacularly, I suppose.
(Nicodemus with his plague habit connected to collapse and death of native American cultures on contact with European diseases ? The Little Ice Age starting when all those Native Americans weren't using fires and in many cases slash-and-char agriculture and ending when the Industrial Revolution started replenishing those greenhouse gases again as an echo of stuff going on in Faerie ? Interesting direction, but not enough ahd data to go on.)
OK, I had forgotten that, and that certainly counts as a boost more than just psychological.
I don't think we have any real evidence to distinguish that from Thomas always having been that good and just going to great lengths to hide it, though. It's pretty clear that he's been doing the "useless party animal" act for a long time to avoid being taken as a threat. (I can see Maggie planning Thomas being as strong a White Court vampire as she could make him, and being born on Valentine's Day seems like it should give him a boost there too.)
My own personal assumption was that the Erlking and the Wildhunt targeted Thomas in DB in order to get even with Harry. However, and here is a wild thought. Maybe the Erlking, like Mab, has been in on this whole Starborn thing from the beginning. Maybe the Erlking knew, not only that Thomas was Harry's brother, but that not only was it important for the Starborn to be stronger, but that it was important for the Starborn's brother to be stronger as well.
What if the Erlking took advantage of an opportunity to go after Thomas in order to give him the choice, hide, flee or join. Getting Thomas to join, meant that the Erlking could influence him in such a way as to give him a power boost.
Then there is the Archive (Not necessarily Ivy), who might also be in on this in her own way. Choosing Thomas to be a Venator meant giving him a pathway to becoming stronger as well.
Of course, Mab seems to think she has rights over, not only Harry but Thomas also. Who knows where that might lead.
Maybe this is bigger than we realize, only we don't see the whole picture yet.