When you say loan, you mean you are paying him back? Do you have all of the receipts for things he/you has paid for?
I did this, (hopefully the right way) First of all I paid my mum back out of my employed wages.
Kept all of the receipts & added it all up for my self assessment (plus fuel, extras like uniform more products etc) & put that as my expenses. It was calculated as 'loss' on my self assessment. As my outgoings were more than my earnings.
I didn't mention that the money came from my mum-as it did initially but I paid her back out of my own pocket.
Does that make any sense?
Xx