Check your state law. Most likely, the bank was put on notice of a breach of fiduciary duty when it allowed the father to deposit the check into his personal account and keep a portion in cash (according to the commentary, obtaining the proper endorsements is irrelevant in this case). Basically, if the son were to make a claim on those funds, the bank would be liable to him for the $1,500.
Teller B is correct. In order for the bank to be considered a holder in due course for the item, it needs to be deposited into an account of the father as custodian for the son. What the father does with the funds after they have been deposited is his responsibility... the bank is only put on notice at the time the check is presented.
Texas UCC 3.307
(b) If (i) an instrument is taken from a fiduciary (THE FATHER AS CUSTODIAN) for payment or collection or for value, (ii) the taker (THE BANK) has knowledge of the fiduciary status of the fiduciary, and (iii) the represented person (THE MINOR SON) makes a claim to the instrument or its proceeds on the basis that the transaction of the fiduciary is a breach of fiduciary duty, the following rules apply:
(2) in the case of an instrument payable to the represented person or the fiduciary as such, the taker (THE BANK) has notice of the breach of fiduciary duty if the instrument is:
(B) taken in a transaction known by the taker to be for the personal benefit of the fiduciary; or
(C) deposited to an account other than an account of the fiduciary, as such (MEANING ANOTHER UTMA ACCOUNT), or an account of the represented person;
In the end, it's all just a bunch of paper....