Congratulations on securing your first client. It is a nerve-wracking time but exciting too!
He could put 20% of his sales receipts away as that will help to cover his tax bill and, depending if he has Class 4 NI to pay, he could have a surplus when his personal allowance is taken into account. It's a good habit to get into from the start so I would encourage him to put something away.
Definitely advise him to open a business bank account, it's much easier for you both to track income and expenses that way. If he's not VAT registered then invoices aren't essential but again they make it easier to track income and are useful for ensuring his customers have all paid. Could you offer to raise the invoices from his quotes as an extra service?
I hope that helps a little and good luck with your new practice.