British Standards for tiling states that dot and dab doesn't leave enough coverage on the back of the tiles. Which for wet areas including showers is 80 - 100% coverage. To prevent failures like yours! You can link the plumber to here to buy the
standards for tiling, BS5385
BS 5385-4:2009 - British Standards for TIling
It's less to do with the method than the result of it. There just wont be coverage on the back of those tiles. If one was popped off, I bet not even 30% is covered on most, and that's not even good enough for tiling in dry areas (walls only) like perhaps a downstairs loo or most of a kitchen. Which is 60 - 80% if I remember rightly.
Plumbers can't always tile correctly just because they've done a few and it hasn't failed before.
You need to document things, perhaps communicate via email so it's all in writing, then perhaps consider legal action if nothing else can be done. I'd also ask for his public liability insurance details. I don't think you can claim until there is damage done somewhere which crazing isn't always classed as depending on the cause. But it'll make him realise the problem isn't going away.
Be professional when you've communicating and don't lose your rag. Don't give him a reason to ignore your calls or emails etc.