Trusses are highly engineered components that are conservatively designed to bear loads specified in your local building codes. The 24" O.C. spacing is conventional for pre-engineered trusses and are quite different from built rafter roofs.
If the load demand had been such that 24" spacing would not work the engineer would have modified the truss plan. As it is, you can be assured your trusses were designed specifically for your home and were approved by the building department for construction. A side benefit to engineered trusses is the economical use of materials. Dad won't win this argument.