It looks like your kid has oral thrush
which is a fungal infection
If he is on bottle feeds it might be the reason and bottle has to be stopped immediately by switching over to spoon feeding.
Sometimes antibiotic treatment may be the cause.
The cheek lesions may also be a part of the fungal infection- muco cutaneous candidiasis
But usually kids don`t have fever
with these lesions.
So have a word with your Pediatrician
. If its confirmed as these fungal lesions, it will be treated with candid mouth paint for the mouth lesions and candid ointment for the external lesions. Duration of treatment will be for about a week. But have him examined before you start the treatment.
Hope my answer helps you.