We did not make an halachic time he has to pay me back, so I dont know if I can even demand it.
I don’t know if we apply “stam halva’ah shloshim yom”, but certainly he can’t push it off indefinitely. In fact, you wrote that it’s six months since he said he’d pay it back.
I agree with Goq. Get it while he has it, unless your intention was to take on a big risk. From the little you’ve written, this doesn’t pass the smell test.
ZD, bais din can allow going to court to enforce its psak.