My fiancé has a trust fund that has not been released to him yet. His mother is the trustee and has full control over the money. My fiancé will become the beneficiary when he is 25, but for now cannot even request statements. Should the mother (trustee) or the son (beneficiary) file the taxes?

Please clarify - is your question: (a) who should take responsibility for completing and filing tax forms on behalfof the trust, or (b) who has to pay the taxes owed?

A trust is usually a separate tax entity which files its OWN tax return (Form 1041). The mother should be one who is responsible to see that this is done.