What you have agreed is the price against a rough specification (I hope you have at least a description of the scope of works) - nothing more.
Unless you contracted over a timescale or built a phrase like 'time is of the essence' into the contract, you cannot force him to complete the work to any particular timescale.
It is up to you to establish the quality of what he does is OK, and to (now) agree with him when he will complete the work. Once he agrees a completion date, drop him a brief note confirming the conversation about the completion date, and see if he sticks to it.
This is standard practice for far too many in the building trade, I'm afraid - have lots of jobs on the go at any one time. It spreads their risk and maximises the opportunity to assure continuity of work.