Are his feet itchy ?
Are there any blisters?
Red under the dryness?
Between the toes are they dry ?Or is the skin mascerated with a red base ?
Hard to tell from the photo but the heel looks that mascerated wrinkly way skin goes when you've had your feet in water too long . Had he been soaking them before this photo?
Patients with diabetes tend not to sweat , it can affect the autonomic nerves (that's the ones that gives you goosebumps and make you perspire) as well as the pressure/sharp/blunt/light touch/hot/cold/ reflex sensations.
Flexitol or Dermatnics are excellent ( Dermatonics has the advantage of Once Daily but most people only apply Flexitol once , it's balm so it's heavy)
There's a foam "Alpressan Diabetic Foam Basic or Intensive" which is good , light weight but effective . It is unusual in that dressings/tape can be applied once it is dry and they will stick 
Fungal infections are hard to diagnose , it's more by symptoms and looking for skin signs . Sending off samples takes a while and you can get a false negitive .
Fungal infection can give rise to dry skin, often overlooked as 'just dry skin; and oddly , on one foot worse .
You most likely won't get a Podiatry appointment at the moment , but most GPs are doing phone consults in the meantime . Try and send a very clear close up photo
Fungal creams are usually a short intensive course and you see an improvement fairly quickly in skin. (Nails take bloody months !)
Standard advice about checking feet daily .
Wash/dry especially between toes
Change socks daily and alternate shoes
Blood glucose control is vital
Good Luck