In the absence of a related diagnosis, like heart attack, muscular damage/injury,stroke, myopathy etc., which would explain the high CPK (1),
there's no specific explanation for the lack of feeling in his feet (2)
causing his falls, so far.
So if we are to make the assumption that (1) and (2) are associated, then underlying causative factors must be explored.
Hypothyroidism, is often associated with high CPK levels, a condition which is easy for doctors to miss, or even worse, rule out because of flawed testing.
There's a thyroid test called Reverse T3 (rt3), which is not ordered, as there's usually no insurance code and most doctors are not even familiar with it. ( Reverse T3 is a competitive inhibitor to T3 and T4.)
This test along with Free T3 and Free T4 testing is the best way
to rule out hypothyroidism, including thyroid restistance (Type 2 hypothyroidism),
Type 2 would be highly suspect, in the presence of leptin resistance and
consequently, inflammation -leptin is similar in inflammatory action as IL-6 (interleukin 6) - would be elevated AND his vitamin D3 levels would be low.
The low vitamin D level, would mean that also B complex and specially B12
would be low. B12 level in methylocobalamin would matter most in his case,because it is the neurological form of B12, which cannot be verified by the standard MMA test. He needs a CSF Homocysteine test for this.
Another common problem is stemming from high leptin levels (which eventually leads to the aforementioned leptin resistance) gradually destroying amylin causing insulin resistance.
If the process happens fast, a form of diabetes called LADA ((latent autoimmune diabetes in adults) may develop.
To summarize, you need to rule out, hypothyroidism type 1 and 2, low Vitamin D3 or deficiency, leptin resistance, low B12 methylocobalamin,
LADA (which is challenging to diagnose in early stages) and tissue magnesium levels -not Hypomagnesemia/low serum magnesium-, which is nesessary for many bodily processes, specially for your son's purposes, muscle & nerve function.
Another imbalance, which I would also suggest you look into is low total cholesterol levels, something that most doctors don't normally consider.
The effects of low cholesterol-even subclinical- on his neurological, skeletomuscular and endocrine systems, could be very serious.
I hope this helps, however, my comments and suggestions are not intended as a replacement for medical advice.