Bastan will only join if you have paintings from...

[gildedtalon]

...Revolving Bridge, right? On all my playthroughs, I've never managed to get him to join if I never get paintings from revolving bridge

[Antimatzist]

Or at Mt Ranro or during the Doraat siege iirc.

[Hirathien]

I certainly never got him by getting the paintings from anywhere else than the Revolving Bridge, might be another place though.

[KFCrispy]

you can get any unidentified painting from any source and just keep talking to him without appraising it until the scene triggers.

[Rooks]

Any "? Painting" can recruit him. The Game randomly decides to change a single "? Painting" into the Ivanov. So, just close out the window and try again. Also, the probability of this occuring grows as the game progesses. By the time you liberate Stormfist, the probability is very high.