£117 is not that much, i wouldn't explain it unless you had to,
how many bank statements are overdrawn in the last 6 months ?
what he's probably better off doing, is getting some money off his parents each week and living off that, so for the next few months he will be saving some money and increasing his balance, but dont over do it, they are not stupid if he saves too much of his wage they will wonder what he is living off

, but if he can loan £300 a month off his parents and use that to buy food, pay for petrol etc, and hopefully his balance will increase by £300 for the next few months, and once you have your visa, he can repay the money..
where are you going to live with his parents? also might be worth his parents writing a letter offering 3rd party financial support should you need recourse to public funds, this could be allowed, need to show evidence they have the funds..