UK Visit Visa, How do we prove self employment? [duplicate]

My husband is applying for a visitors visa for the UK. He is self employed, or employed under his mother, working in their farm in SA. He gets regular income through cash and through his bank account (his mother wires money in his acc)

How do we prove his self employment? His mother does not have a business or a company, its like when you hire someone to be your gardener kind of 'employment', but not as a once of, but long term.

They live in a very big farm that needs daily maintenance and he does all the work. Would this even be called self employment or just under the 'savings'?

Do we just need his mother's confirmation letter and bank statement? as well as his bank statement for showing income for 3 months? 6 months?

Thanks.