Here's some sources you can use:
http://ancienthistory.about.com/library/bl/bl_smithhistoryofgreece17.htm
This one talks about how Sparta's former territories were divided between Jason and Thebes, and points out not only his ambition, but the relief the other Greeks had at his death.
http://www.archeobooks.com/servlet/the-432/Jason-of-Pherae,-A/Detail
An out of print book about Thessaly around this time period, could be a good source if you could get it.
http://th-th.facebook.com/pages/Jason-of-Pherae/115990598449025
A facebook page about Jason of Pherae, tells about how Isocrates pleaded with him to unify Greece (always a good sign when when a philosopher, who are usually pretty independent, wants you to unify is country), and claims by Jason that he had six thousand of the best mercenaries, and also that he went through the same things his troops did.
http://books.google.com/books?id=Fz...Q6AEwAzgK#v=onepage&q=jason of pherae&f=false
Here's a GoogleBooks thing about Jason of Pherae, detailing the battle in which he became one of the most powerful rulers in Greece, and like all the other sources, pointing out his diplomatic and military skill, as well as his ambitions.
http://asmalltowninlaconia.tripod.com/ASmallTowninLaconia/history/theban_hegemony.html
This source repeats the earlier info, gives a somewhat concrete number for Jason's number of troops (6,000 cavalry and 10,000 hoplites), and points out that Jason took the legendary Thermopylae.
My own thoughts: Don't know much about Ancient Greece, didn't know about this guy before looking at the wikipedia article, but from the sources I gathered in about 20 minutes, he seems to have been a Proto-Philip of Macedon or mini-Alexander the Great.
In my experience, whenever a historian says that a historical figure has "boundless ambition", that's a good sign for what they could do, if so inclined. William the Conquerer, Alexander the Great, and other world-changing rulers all get the phrase "boundless ambition" stuck to them, and few others do. So if Jason of Pherae has that phrase describing him, he's probably an influential person.
Plus, didn't Philip of Macedon and his son Alexander use the Thessalian cavalry to great effect in their campaigns? So a ruler born in Thessaly, commanding the loyalty of those horsemen, could use them to even more massive effect.
