Your not a registered charity, that's a vat number on your invoice.
Your in business to provide a professional service at a fair price and to date that's exactly what you have done.
Its all even at the moment, don't let your heart rule your head on this one.
Weigh up pro's and cons, put your points to him, if he's giving off bad vibes, walk away, and tell him why.
But do not go too deep into his pocket, that's when it comes right back and bites you on the proverbial.
My thoughts for what its worth Skelts.