Hi !
I welcome you to the forum.
To begin with, English is not my mother tongue, please forgive me if I make some syntax errors

I owned a PSR-E413 for some time 6 years ago. This "low-end" keyboard is good for a complete beginner, to make sure that playing the keyboard will interest him. It has some good quality sounds, well-made styles... The keys are normal size, which is nice when you want to switch to another model.
But if playing the arranger keyboard pleases your son, very quickly, he will feel cramped (same for the E373): the keyboard itself is "plastic", bordering on a toy.
If you have the opportunity to have a more elaborate model (not necessarily talking about the PSR-S600, S700 and S900), but at least a PSR-670, 770, 970, etc..., the musical game will be much more pleasant.
To sum up, the PSR-E413 (which I know better than the E373) is a good little "low-end" keyboard for beginners, and has the advantage of being really cheap (in France, around 100 to 130 euros ). But you quickly reach your limits when you want to evolve in the practice of the arranger keyboard.
Hoping to have helped you a little