Final answer:
The nurse should notify the primary care provider and ask for a referral to a mental health professional.
Step-by-step explanation:
The nurse's next action should be to notify the primary care provider and ask for a referral to a mental health professional. The client's difficulty concentrating, feeling short of breath, and experiencing sleep disturbances may indicate psychological distress, which could be related to the stress of the bleeding duodenal ulcer and its impact on his daily life. By involving a mental health professional, the nurse can ensure that the client receives appropriate support and treatment for any potential mental health issues.