Hi Nina
You dont say if he is VAT registered or not, although Im guessing not.
Invoicing is still a viable option, but rather than manually raising one via the computer each time, then why not get a duplicated invoice book with sequential numbering - one copy for his client, one for the him and you doing his books. HMRC would prefer this to loose bits of paper! He can buy from any stationery shop or he could have them printed with his logo etc. Dont forget to add the VAT/include his VAT number if he is VAT registered.
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Joanne
Winner of Bookkeeper of the Year 2015, 2016 & 2017
Thoughts are my own/not to be regarded as official advice,which should be sought from a suitably qualified Accountant.
You should check out answers with reference to the legal position