This will not work at all because this model got to much polygons. The performance within OMSI is not great its even not good at all, with this model it would be unplayable. It would maybe work with ETS2 or some other bus simulator but not with OMSI.
You would need to highly customize this one to make it work within OMSI. Like reduce polygons. Doing some new textures etc.
Most of these models just even don't have any interior at all. It just looking good from the outside.
It would be easier to learn Blender or just pay a guy that build one for you or your son. Im sorry for you and your son.