Oppositional Defiance tendencies

Hi Nicholeen,
This is a follow-up question about ODD tendencies with adults. When the children ask their father questions such as, "Would you like to build this Lego rocket with me?" Or "would you like to play chess with me?" He would answer "yes, I would like to but I'm busy right now". Then the children usually would ask back, "When then?", and his answer would be "I don't know", so it's then just forgotten and it never happens. What could we do to so that the activity actually happens? Thank you!