The best thing you can do today is put in writing all the things you are not happy with. Tell him you are withholding final payment on these grounds and he has 14 days to rectify the problems or you will get another contractor to do so, possibly at his expense.
i.e The use of adhesive as a grout is not suitable,
the large dips in the plaster work,
the trowel marks in the plaster work,
the varying sizes of the grout lines,
gorut lines not meeting up,
the
tile adhesive on the face of the tiles,
the over tiling of the towel rail foot,
the tiles not going into the door threshold,
the plaster splattered on the door frame.
And that's just from the pictures you uploaded!
You maybe able to find a legal template for this letter on the internet, maybe from citizens advice or trading
standards