attitude or attitudes?

The difference between the two boys is their attitude*, *what is especially obvious.

Question 1: Is the attitude* being singular here correct? Shouldn't we use *attitudes, as, definitely, the two cannot share the same.

Question 2: Is what* here correct? If so, how is it functioning here? Can it be a relative pronoun? Or do we replace *what* with *which instead?

Please help. Thanks!