Hello,
From the symptoms boils, abscesses and erythema multiforme need to be ruled out. Erythema multiforme is a skin disorder due to an allergic reaction or infection and can effect children and young adults. Skin biopsy can be helpful in confirming the diagnosis. So please consult a dermatologist and get it done. He also needs a culture of the discharge from the bumps to rule out any bacterial infection.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.
Sounds like a fungus or reaction to things he is exposed to alot. Maybe start removing certain pesticides or other agents, one at a time for a couple of months. Once he stops getting the sores...you will know the culprit. He could go through allergy tests too, if he has insurance coverage.
You didn't explain what these sores look like. Are they open and oozing? Crack like with bleeding? Dry and scaly? Blister like? Peeling skin and itching? May need to describe in more detail.
Good luck farmer man.