Why Should My Son Do His Project On The Lock Ness Monster?

Is this your assignment? Or your son's?

The teacher likely will be interested to know that the scope of the project fits within the guidelines of the assignment, and that your son has sufficient interest and motivation to carry it through without having someone (you) do it for him.

The project could conceivably range all over the map, investigating possible origins and natural history of large water-dwelling creatures, origins and natural history of the loch, the available evidence and what people (experts?) say about it, methods of searching for Nessie and their results and limitations, media coverage through the years, estimates of possible food sources for Nessie and the amount required to sustain the possible population, possible ways to fake the photographs (given technology limitations at the time).

There's enough meat in a project like this for many months' work, if your son wants to go looking for it and if he pursues it in the directions it can lead. The ultimate convincer will be the ideas and enthusiasm projected by your son.