M.K.
I am a podiatrist - or was for 12 years in the UK, until I moved to the USA.
There are just a few things this could be
1. as the first lady said - sever's disease - not common, but not too uncommon.
2. Tarsal coalition.
3. plantar fasciitis.
you can google all three, to see which matches his symptoms, then see which treatment to use - all three have a similar treatment plan, of steroids, casts, anti inflammatories, rest and stretching etc