I presume you're not really 'writing about smoking'.
If you want a 'real' heavy smoker, you are going to have to think about the permanent smell of tobacco on his clothes, his breath, his moustache.
To a non-smoker these can be extremely annoying and off-putting. In a car, in a lift, in a house or flat, hotel rooms, buses, trains, restaurants, in theatres, pubs, cinemas...
It was pretty grim, even though 'accepted' at the time. But not everybody chain-smoked.
Being a heavy smoker would have an impact on his behaviour and on those around him.
He'd always be looking for an excuse to wander off, to light up. His fingers would be nicotine stained, depending how he held his cigarette, or his hand. If the smoke is going vertically from the cigarette he
may avoid the nicotine stained fingers, but if he smokes right down to the filter or to within burning distance of his fingers, and lights the next one off the stub...
I knew someone who had burn holes in their vest through lighting up before getting dressed. Yes- it's the first thing a heavy smoker does in the morning, and usually the last thing at night, too.
No idea what your story is about, but have you considered making the MC the non-smoker? Or at least just an occasional 'puffer' instead of a 'heavy'?
You'll know what fits your story best so I wish you well whatever course you follow.
Good luck.