My parents have also been caught out (more than once!!) by paying too much deposit in advance. Long story, but to help you avoid the red flags:-
Firstly, you should absolutely 100% be asking for a detailed quote - with itemised costs for materials.
2/ Have you been involved in choosing any of the items? Asking because the quality of these can vary quite significantly… and especially for tiles. Even if you are going for plain white standard looking 6x6 tiles the thickness and finish does vary quite a lot.
3/ have you had other quotes? Another reason why you need an itemised quote - to compare prices. Good workmen will not be cheap, and will usually be booked quite a long way in advance. Suggest asking on a local FaceBook group for recommendations, and maybe even speaking to people who they have done work for (as on FB, their friends and family will often recommend- not just previous customers)
4/. You should not be paying in advance for any labour. Red flag
5/ Materials - any reputable builder/plumber will have a line of credit at builders merchants or suppliers for standard items. That means they do not pay in advance, and you might even be paying him for the completed job before the builder needs to settle his account.
Special tiles etc (that come from a ‘posh’ shop, or need to be specifically ordered) are a different situation and I would expect to pay for that upfront.
5/ If it is a small/ start-up company without access to credit then definitely make sure you pay the supplier directly, and arrange for delivery of items direct to you (ie/ do not allow builder to collect on your behalf). Then in worst case scenario, if things go wrong and you need to employ someone else to either do the work or finish any badly done work, at least you are not out of pocket for the materials.
If you have issues with the work not being completed satisfactorily, then only having a small amount left to pay will not be enough incentive for many ‘cowboys’ to come back and do snagging.
In my opinion - 40% upfront is very high, much too high !
That suggests he has cash flow problems - another red flag, and not something I would want to risk.
good luck