That'd probably be a Jump skill check or a Dexterity check, depending on the DM's discretion. Possibly some sort of other check to verify that he manages to grip the monster enough to remain upon it initially (as in doesn't just splat on him and slip off). That would probably be after the dmg rolls (Assuming contact), and probably only if the DM was anal.

so, it'd be something like:

roll to land, attack/dmg rolls mentioned above, roll to determine if your landing "stuck" , rolls to continue to stay on (the ride-type rolls mentioned)

Given the different physics, I would put the "sticking" roll different to the 'riding" rolls, but that's just me personally.

but yah. your dm's just being lazy/a dick. There's no reason within the rules or physics that this shouldn't be allowed.