Being a parent is hard, harder for some than others, but in any case hard! Im not a parent just yet as im only 21 but ive helped raise my cousins and know how it feels when they dont want to do something!
Ive had embarresing moments like trying to stop my cousine from letting himself go on the pavement, and crying for his mother while people starred at me!

Children are hard headed however i got a question to ask you. Did you ask your son what the problem is? why he is doing this? He may be young but these kids today have strong ideas!
Is it just because he misses his mom or maby something at school which is annoying him? Talk to your children sometimes it is something very small which is trobeling them!
(I am not saying that you do not talk to your children, this is not an offence in any way)

I remember my aunt making a pakt of some kind with my cousine, like if he goes to school she will buy him something he wanted (dont remember wat)! Try this you never know!

Anyway hope that the prob will be over soon! Good Luck!