Isn't it just so easy...
...to put things down to the unprovable.
It's amazing how some people get ideas into their head and start associating them no matter how illogical and the fact there's a lack of evidence to prove the point.
Someone lost their son and their wife that smoked heavily within 3 months of each other? I lost both my grandparents within 2 weeks of each other last year but it was hardly a ghostly incident, it was just unfortunate that they were both at that point of their lives where they didn't have long left. For some people this is earlier, some this is later in their lives but ghosts have nothing to do with it.
He says he's seeing things and smelling things at night particularly? Well yeah, people are prone to this when they're tired, when they're woken up from dreaming and stuff.
As for the text messages, he himself just says he presumes they're from his wife, it could be someone playing a prank because they know how gullible he is! Here's an idea, get your phone provider to block the sender if they can't tell you who it is. Inform the deceased wife's provider that that number is no longer in use if you're that concerned.
It's pretty clear this is a guy who has suffered two tragic losses but is also extremely gullible, the combination leads to him believing what he wants to believe whether consciously or subconsciously, either way it's obvious there's nothing in this story to show it deserves any amount of credibility and certainly nothing unexplainable with simple common sense.