Oral thrush can be quite sore. Does it look thrushy? i.e white patches, red shiny patches or is it just ulcers?
Taken from the NHS choices page I linked to up the thread:
"Symptoms of hand, foot and mouth disease
Symptoms of hand, foot and mouth disease usually appear three to five days after infection with the virus. They last for 7 to 10 days before disappearing on their own.
Some people with hand, foot and mouth disease do not develop any symptoms.
If an adult develops the disease, their symptoms will usually be much milder than those of a child.
Early symptoms
The first symptoms of hand, foot and mouth disease include:
fever and feeling unwell,
loss of appetite,
sore throat, and
small red spots in the mouth, throat and skin.
Later symptoms
Mouth lesions
After one or two days, red spots in the mouth will develop into painful ulcers, particularly around the tongue, gums and inside of the cheeks. It may be difficult to eat, drink and swallow.
Skin rash
Any red spots on the skin will turn into a non-itchy rash over the following one to two days. The spots are flat or raised, sometimes with blisters, and smaller than chickenpox sores.
The rash develops on the palms of the hands, the soles of the feet and between the fingers and toes. In some cases, spots also develop on the buttocks and genitals."
Looking at that it seems the skin rash will come in a day or two.
Poor DS 