Should I tell him?

My fiancé and I have talked and he knows there is a hiiiiiggghhhh chance of the baby we are expecting not being his, he says he doesn't care that he is going to be his daddy and we are going to be a family no matter what. The other guy has no clue and has his own family (girlfriend&daughter)...should I tell him that it is probably his, or just leave it as is since my fiancé wants to build our family together regardless of the results?! I have mixed emotions on this situation.  -no rude comments please since I didn't tell the WHOLE story about why it might be his and not my fiancé's. 

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