Only the people who sign the tax return are directly liable for the taxes owed on that return. That said, if he files Married Filing Jointly with you now, you both need to understand that any overpayment/refund will be kept for your indivudual back tax liabilities. He would be able to file for Injured Spouse, however, this process is complicated and may not result in him receiving his portion of the refund. You may choose to file Married FIling Separately, in which case he would owe whatever he owes or get whatever refund he gets based on his information alone, and you would do the same. Any refund you may be due would be applied to your back tax liabilities.
Since you do live in a Community Property state, the IRS may issue collection actions on your husband's accounts and wages, as well as yours. The IRS is much less likely to take any kind of collection actions if you are on a repayment arrangement or Installment Agreement to fully repay your back liabilities. You can either attempt to resolve this yourself by contacting the IRS or by hiring someone to represent you with the IRS.