I would have thought that the progression, from first easy ones to gradually getting harder, would, to some extent, help with learning, and I agree the 'kinaesthetic' feel, or 'muscle memory' as it's sometimes called, can also help.
As a teenager I tried to teach myself trumpet, from a tutor book, but never got much good. Trumpet is difficult in that, at first there don't seem to be a logical reason for the combinations of valves to achieve different notes, but presumably he is well on top of that by now.
Do you have a piano or keyboard available? There will be at school anyway, and I assume he in in secondary to be doing Grade 5? One of the hardest things to absorb at first is why, on a keyboard, there is not a black note between E & F, and between B & C. As in so many things, UNDERSTANDING is important, as just being told the 'rules' isn't much help if you don't understand why.
Do you, and he, know and understand the 'harmonic series' and 'cycle of fifths'? :
On an instrument such as the cello, if you pluck the C string, the string will vibrate along it full length to produce the note C. BUT it also vibrates in half, to give C an octave higher. It continues to sub-divide, giving different notes, and this provides the basis of western harmony.
This website explains about it, but you can probably find lots of similar things on-line: en.wikipedia.org/wiki/Harmonic_series_%28music%29
The diagram can be enlarged by clicking on the 'double box' sign.
So it is a characteristic of natural physics the way sounds are produced, which is why 'western' music sounds so different from Oriental or African.