Your dad has features of Dementia which are
2)Apraxia(difficulty performing learned motor tasks.eg operating oven or microwave in your dads case)
3)Speech abnormality like Word finding
Usually dementia is more common after the age of 60 although it can occur in younger individuals.Take a second opinion from a Neurologist
or a Psychiatrist
who will be able to evaluate him thoroughly with extensive Cognitive testing & Neurological examination.
One of the explanations why the doctors find his examination normal when the history is suggestive of Dementia could be he has mild dementia, high intelligence or education or depression.
Considering his relative young age an imaging of the Brain with CT or MRI is indicated. This will help rule out other causes like a slow growing tumor or other intracranial pathology like Normal pressure hydrocephalus
or a subdural hematoma(although the last two are less likely).
The commonest cause of Dementia is Alzheimers Disease while other causes are Frontotemporal dementia
,Dementia with Lewy bodies,Vascular dementia
Hope this helps. Kindly get back if you need further assistance.