Do you exclusively control the github repo? If so, cut off access and don't give it until payment is made in full. You have an implicit copyright on the code, and he has no right to it until he's paid you in fill.

I'd go a bit further and tell him that if he doesn't pay you within 15 days, the "interest" is $100 per week. No reason you should not get interest on your debt.

Just FYI, I always let customers see "features", but not code, until they've paid in full.