Hi thanks for your question.
Your 42 year old son is having dizziness
and fatigue with heart rate
of 40 beats per minute.His symptoms of dizziness and fatigue are due to low heart rate,though his blood pressure is normal but at this heart rate,vital organs of his body espicially brain is not getting adequate oxygen,that is why he is experiencing dizziness and fatigue.I would suggest you to consult a cardiologist
who may be able to evaluate him for cause of his low heart rate.He may be having sick sinus syndrome
(a benign problem) or thirs degree heart block
.He may need pacemaker.
Hope this answers your question.