If he insists on those conditions (and they seem, on the face of it, reasonable) then I would also insist on some of my own, particularly to do with timing. I would suggest something like "work should be continuous for five days a week and last no longer than (say) six weeks (or whatever you deem appropriate). For each working day that the job extends beyond six weeks a penalty of 5% of the total price will be payable." I would get that in writing as well.
Builders have a habit of disappearing part way through a job, usually to start another one. A relative of mine has recently seen his job begin, the builder to take two weeks off (which was agreed beforehand) and has now developed a number of medical problems which are preventing him from working (despite having been seen in the town collecting materials from a builders' merchant).