I think your paladin has behaved pretty well, really. I mean, look at a cop. If you were a cop and you saw your buddy Joe waving around ... a sack fulla cocaine, or something.. you're not too likely to go tell your sergeant about it, or whatever.. you'll arrest him if he refuses to fork it over -- and if he's -armed-? Well, if you're a lawful good cop who is on duty -- and paladins are never off duty.
While there's a lot to be said for maintaining party cohesion.. I as a DM and player have always always preferred players to roleplay well rather than maintain an adventuring party just because they feel they should.. were I the DM in question, I'd have no problem with your paladin killing the aasimar (not that I think you should do that, since you should try to work it out first), if it was in-character and reasonably realistic activity. Character sheets are cheap, and it's never hard to work another character into a campaign.
Don't get me wrong -- I really prefer when the party gets along, but they should not get along just because the puppeteer forces them to do so. My worst campaign issue with this was a guy who was always stealing from the party, always.. and then when they caught him, which they always did.. they never really -did- anything about it. They didn't kill him, or expel him from the party.. they just said "Oh Waldorf, you rascally halfling. Do not pick my pockets again, ho ho!". Well, they -finally- got rid of him, but only because he got himself killed running away from them after a theft.
So my point here is, play your character as you feel you should, and the aasimar should do the same thing, and hey. Wouldn't be the first paladin to have to deal with the corrupting forces of foul evil magic.
Oh, and I do definitely think that buying the whistle from him is the best way to go. Maybe some lectures, too. I wonder how long it would take before he got sick of long lectures about the evils of undeath and necromancy. Really long ones. With a collection plate.