You may find helpfull PA DOR sales Tax information - http://www.revenue.state.pa.us/revenue/lib/revenue/rev-717.pdf
The services your husband is doing are subject to sales tax.
Let me know if any details are needed.
The sales and use tax is imposed on the retail sale, consumption, rental or use of tangible personal property in Pennsylvania.
As a subcontractor, he may or may not involve in the retail sale - this will depend on the specific contract. If contract states that the general contractor is responsible for collecting sales taxes, than he doen't need to do so.