I'd tell him nicely but firmly that the workmanship is too poor to leave the way it is, that you want it redone properly and that the tiles need to be taken off immediately before the adhesive fully sets and possibly damages the substrate. If he refuses then get those tiles off: you have detailed photographic evidence of his work so the poor quality can't be denied and you are running out of time. Withholding payment isn't a good idea though, you must try to resolve this with him before refusing to give him any money (perhaps he could pay for someone else to rectify the job and receive the difference).
Unfortunately most good tilers are booked up in advance for quite a while (I've a year waiting time) but the difference is well worth your patience. Beware anyone who can start immediately without a very good reason and always ask to see examples of his/her work. I wish you all the best.