My father has started exhibiting symptoms of asthma since last couple of months. We consulted a gastro and he was diagnosed with esophagitis He is on medication of motinorm, and neksium for his gastric troubles.. but his constant coughing has not reduced. He has been taking levolin nebulisation of and on.. can someone please advise on what could be going wrong here?

So... you haven't taken him into a doctor since this started why?

Diabetes really doesn't leave any part of the body alone, and he's not getting any younger. We know nothing of his diet or activity level, 2 extremely important factors. You can't just go online and ask about specific drugs and symptoms without a doctor seeing him in person to evaluate all of him. We don't even know what tests he's had for this.
Try another specialist, a respiratory one.