You've waited a whole
month?! I feel for ya, man. I'd be climbing the walls if this happened to me. Did he even contact you in the meantime, or did you have to chase him?
Also, when he presents it to you in the shop, ask to look inside and check that it looks like it's been put together ok. I'm talking about obvious things such as actually containing the parts you've been billed for, CPU cooler is sitting straight and running quietly, no screws missing, wires hanging out etc.
Then ask to see it working. Only after you're 100% satisfied all is ok should you even consider paying him for it.
And next time, seriously come to us! We like nothing better than helping out fellow enthusiasts build their PC.