It would raise alarm bells with me. I have a number of friends in the trades, and also in trade-supply companies, and materials are typically bought on anything from 30 to 75 day terms (probably closer to the former in these straightened times!).
So, if he needs to buy the materials upfront and pay cash for them, I'd be wondering whether he had run all of his lines of credit at the suppliers.
Having said that, a deposit is reasonable, but probably just a good-faith deposit of maybe 10%.
In the last few months, we've had gardeners, window replacement, decorators and carpet installers, and have paid 100% on completion.
Maybe you should be direct and ask him why he needs to pay for materials up front?