Ray -
I work in PA. The law can be interpreted a couple of ways. If it is repair work, he can charge tax (he almost has to because he will be taxed for it at the end of year) because it can be accounted for in a different way depending on how he does his taxes.
However, if this is a rough-in by a plumbing contractor, he cannot charge tax on labor.
He can on materials, but he has to show you that he is simply passing the sales tax on - for example, T&M billing, labor is $100.00 and materials (that he purchased) were $100.00 plus $6.00 for tax (total of $106.00). He cannot tax you on top of the $106.00.
I would request one of his material invoices to see if he is merely passing the tax on via an itemized value on an invoice, or if he is in fact purchasing materials and paying tax on them, then charging a sales tax on top of that - illegal - can only be taxed once.
Also, someone on here mentioned a 1099. If he is Incorporated, no 1099, but if he is a sole proprietor or LLC, must receive a 1099 after $600.00.