Sun in his eyes? He's at fault. If you know you can't see, you shouldn't be driving. The law basically says you're responsible for driving safely, and driving when you can't see isn't driving safely.
Now, if he wants to go to BMW and blame them for making a car that'll allow him to drive when he can't see...
Oh, and definitely get your car repaired by a smart dealer, not an insurance company's local shop network. They dealers are the ones that know instinctively whether something's bent slightly out of spec. Anyone else will have to research it.
Let us know how the repairs go... we hope you're uninjured, but we also want to know how well the car holds up after being rear-ended with so little visible damage. If your steering's affected, there's probably damage to the suspension.