Your contractor (boss) should provide you with a monthly statement each month, showing you how much tax he has deducted, and paid to HMRC on your behalf.
It's your contractor's legal responsibly to provide you with this information, and it is your legal responsibly to keep that information safe.
If you have been told that tax has been deducted, then you must make sure to get the deduction statement showing exactly what they have done.
If there are ever mistakes on your tax return, the first thing HMRC will ask you for is evidence of your tax deductions. If you can't provide this and your contractor has disappeared, then you may find that you need to pay the tax that's been deducted from you again!
This would be a very harsh way to learn the lesson, so make sure you get the statement. You probably wouldn't just lend a stranger 20% of your pay without having a record of it, and this is no different.
If your contractor refuses to give you a statement, you can show them this link: https://www.gov.uk/government/publications/construction-industry-scheme-payment-and-deduction-certificate, stating clearly that they must give a written statement to every subcontractor they make a deduction from, and that they must do this within 14 days of the end of each tax month.
If they still fail to provide you with a statement, then you should consider reporting them to the CIS Helpline at HMRC.