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There are a few things at play here....your Son was his primary caretaker when he was a baby and so a wonderful bond was developed. The other issue is that kids go through stages of being clingy with one parent or another. This can change rapidly and even go back and forth from day to day. I think your Sons wife should not take any of this personally and provide empathy and understanding around it.
Also, your son is away from home as you say for work and that can cause distress for any young child in the toddler years. Before each trip, have him remind his son that Daddy goes away and always comes back. It will still be tough on the him while he is away but in time it will ease up as he continues to grow and mature.
While away do they have the ability to view and talk with one another via the computer? This is a suggestion that may or may not be helpful as it could make him more sad, but it also has the potential to make him feel safe as he can see his Daddy and feel reassured and maybe Daddy can even read a short book or sing a song or two. I would keep these visits short and sweet if they decide to try this.
The thing for everyone to remember is that this is normal for him and part of his age and the good news is that he has such a wonderful bond with his dad. The best thing mom can do when he is sad is to reassure her son that his Daddy loves him and will be home soon. These moments aren't easy at all, but with love, patience and reassurance all can get through this.
I look forward to hearing your thoughts.