If he is working for him all the time, then he should really be an employee, but this is rife in the building trade.
If he is self employed then he should be setting his own rate of pay and hours and providing his own tools, but that often doesn't happen (which is why it should be employment!). It should still be over NMW though, which although it doesn't apply to self employed people, he would be daft to work for any less than that!
Remember that he won't get any holiday pay or sick pay if he is self employed.
He needs to register as self employed, and also as a sub contractor, and then the contractor should take 20% off every payment, and give him a statement once a month that shows the tax that he has kept. The contractor then pays this to HMRC once a month. (as PP said, until he is verified, the contractor will have to take 30% tax off him).
When your DH does his tax return, he enters onto there the amount of tax that has been deducted, and it is set against what he owes. Subbies who do 100% CIS work, often get a tax refund at the end of the year.
If the contractor is not retaining the tax then he is not doing it correctly, but if that happens, then make sure that your DH puts away 20% of everything, in order to cover the tax due, which will be twice a year once he is in the HMRC payment on account system