How do you get your fourth grade home schooler to actually want to do school work without-?

Having to threaten to send him back to public school if he doesn't crack down and study. I home school my son for various reasons. The top two being that in third grade he missed A LOT of school due to illness and the second being we had a problem with a couple of "bullies". But it seems every time we start our school day when it comes to the writing part he wants to just stare into space. I'm not sure if I am over-whelming him having all the subjects everyday or should I space it out, I'm a newb, any advice would be appreciated!!!!