Can I ask, How is a model classed as a scratch build if the doors, the windows and the shingles are bought? Ain't that really a kit bash?
There really isn't a fine line between what is scratch, what is kitbashed, and what is just kit built.
Technically, you could make the case that neither my models nor Bernards are scratch built...even though I make my own trucks and hardware...only NBW castings, couplers, brake wheels, and wheelsets are commercially produced.
The NMRA has a guideline...I don't recall, perhaps 25% with commercial castings such as windows and doors acceptable. I prefer to make my own windows so that they are spot on for the prototype.
Generally, it seems that scratch is when you assemble loose materials on your own, find or create your own plans, and produce a model. It seems that the windows can't be part of a commercially produced wall section, or then it is kitbashing. You are creating your own rather than modifying something commercially produced.
His building would be considered scratchbuilt by most people. It is also a nice start, for it has built his confidence, shown him the satisfaction of creating it, and taught enough lessons that he'll be able to tackle larger and more complicated projects. Additionally, unlike my first attempt which involved producing my own windows from styrene strips, his won't being going on the trash heap any time soon.