We also live on a farm.. My boy will use the toilet if need be, but will wee on the grass if it s at all possible.. It could be 10pm and he will be there at the front door trying to get out to do a wee.. He has no shame!! lol  No matter where we are he will flop it out.. Ive now got him to at least go behind a tree, or somewhere where no one else can see his willy.. But if I tell him to, he will use a toilet..  Oh but he LOVES the trough in the mens toilet!!  Such a novelty LOL Im not sure what you could do to encourage the toilet more though.. maybe get Dad to use it a bit more for now and see if he will try and copy???

Hi we live on a farm to our son started tiolet training on the grass, he loved copying daddy, we had no problems with the training until he started day-care he pulled his pants down and went on the grass the teacher asked us where he goes to the toilet at home as he went on the grass.
But they(teachers) understand that we live on a farm and that children want to 'do as daddy does'

 They learn were it is allowed and were it isnt allowed. My son is now 5 and he only wees out on the grass at our house and Nanna's farm.
He learnt this by 2.5-3yrs.
another thing that was , he done a poo on our lawn, we told him that he is only allowed to wee on the grass and poos go into the tiolet, he only done that once LOL

My Daughter is wanting to copy Dad and big brother know  and she tries to wee on the lawn.

KIDS WILL BE KIDS

You could always plant a lemon tree.. I don't know what it is about farms... But when ever there is a party at my folks place her brothers always go out in search of the lemon tree... :)  And my parents have three toilets.. Ok they don't really have the water to have them all flushing away but really the lemon tree... LOL

Also as strange as this sounds... Pee is good for growing tomatoes.. I have no idea why or if I would even eat a tomato that has been peed on to help it grow.. But..... :)

You could try buying plastic flies... He might enjoy trying to kill the fly in the toilet.. Or just a sticker on the inside of the toilet.. Something that won't damage or block the toilet if it gets flushed.

Hi
Grass works lol...we taught my son on the grass....was sooo much easier and in this time of drought...gets to water whatever grass is there
You will find when you go out he will use the toilets as they are a novelty.
Be happy he don't pee inside....beats me how they miss lol
Luv Deb