I don't think he did. In fact, he pretty much told you the exact track you need to take to get it. Only the highest ranking Knights of Solamnia are eligable. If you want to go for it, you have an explicit path to take to get it.
And likely for a reason. (I'm assuming the following as an example on how I might tackle this if I'd be running a Dragonlance Chronicle-- likely there are a million other reasons)
If he's prepared to run a Dragonlance Campaign, he likely read up a bit on Solamnic Knights, build them into the plot to get an authentic "Dragonlance-Feel" across and might have some rough ideas for adventures that include Solamnic Knights set aside.
Now you have a player who wants to play the friend of a Dragon (likely riding a Dragon aswell) but explicitly does not want to play a Solamnic Knight?? Not only does that player infringe on the possible character development of a Solamnic Knight that might exsist in the Chronicle, but the DM also has to come up with a whole new plot/story/background for a friendship of chromatic Dragon and Player.
One that goes beyond and above the organisation that already exsists in the setting to cater to such character concepts and he also has to include it in the setting without violeting the worlds integrity.
Thats alot of work for a single players fancy and it likely wont improve the quality of campaign. Sadly, many players are content to "just get the PrC" and leave the work behind it to the DM once they are "in".
It's also rather boring and dull work, because a concept for "Dragonfriends" or "Dragonriders" already exsists in the setting.
One that has quite a bit of material published for it too.
So by telling your players they need to be Solamnic Knights (and experienced ones at that) to take the class you can combine the players wishes ("
would benefit very much from this prestige class") with the need to build a good campaign.
I think if your playing an Aasmiar paladin on Krynn, enough of the basic standards of Dragonlance have already been set aside that one more wouldn't matter.
I disagree. Playing exceptions to the rule can be very rewarding (if the DM agrees and has a story set aside for it).
But it does (IMO) pose an additional hindrance on the Platinum Knight, since being Aasimar on Krynn likely already is a major plot and an exotic feature. Combining it with a PrC that isn't heard of in the Setting than, might turn a cool character with a unique background into one that is just beyond suspension of disbelieve.
As a DM, you'll also want to cater to all your players. So if one person on the table already has the exotic Race, you'll likely want to save the exotic PrC for someone else...
If I had an Aasimar character in a Dragonlance Campaign, I'd strongly recommend a very, very typical Dragonlance-PrC to the player, in order to firmly tie the character to the world.
