Think of the type of injuries farmers get, falls, crushes, equipment entrapment, exposure (heat stroke/hypothermia), grain suffocation, fumes, burns.

He needs to safety the farm. A cut can wait, any serious injury he's going to need a ride/help. So satellite phone? Kill switches on his equipment? Tractor distress beacon? GPS? Clearly mapping the farm and knowing what land location he's in at all times, knowing the land well enough to give clear directions.

And a stapler is a terrible idea if he doesn't know how to clean a wound. Closing a wound before its clean can increase infection rates. He can control bleeding with his shirt if he needs to , the cut will most likely be filthy anyways so he'll have to come in for cleaning and shots.

Knowing his chemicals and labelling them clearly for EMS. What kind of farmer is he? Does he work with pesticides?

If you're American... HEALTH INSURANCE. Making the farm safe will lower costs. If you're set on making a first aid kit...sterile water, tweezers, polysporin, gauze, CAT, Kling, shears, SAM splint, sun block. I can't really see a farmer picking this up everytime he needs to change vehicles. This is for minor trauma, if you're concerned for his life none of this will really help except the TQ.

This one is cheap right now http://www.amazon.com/gp/product/B0080XQZ12/ref=oh_aui_detailpage_o01_s00?ie=UTF8&psc=1

Grab what you need for him and throw the rest in your trunk. I think its a great deal and I have the same kit in my car. Add some bottled water for dehydration and quick wound cleaning and you're good to go.

The best thing to keep him safe is to have a farm hand ride a long with him.