does he have a litter tray? although you may want him to go outside this is what we did for our male and it worked a treat.
We firstly bought a litter tray and some kitty litter but instead of just putting kitty litter in the tray we mixed it with dirt from outside (so he got used to doing his business in the dirt) then after about 2 weeks we popped the litter tray just outside the back door then after a week completely removed it.

Since then the only accidents in the house he has had is when he has been left inside and we have gone out.
HTH