It is always best to surround any child with as many people that love him and care about him as possible which might include having more than one adult male role model in his life.  Truly, there should be room for two adult male figures if each are willing to be actively involved and to share the responsibility.  In order to avoid some confusion however, calling each of the two men by different names might make the situation less confusing for all involved.  For instance, having him call his real daddy, DAD and call his mother's fiance a combination of his name and the word DADDY,  for example, Daddy Mike or Daddy Jim will help to keep the two men seperate for this little boy and make communication easier for all involved.  
The mother should take the opportunity to foster a connection between her son and both of the men.  If for some reason there is an inability to come to an agreement regarding the sharing of this responsibility, a mediator may be able to provide assistance to come to a workable solution.  
Encouraging both men to be a part of this little boy's life can only provide a richer, more meaningful future for all.    

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