I'm doing my deceased son taxes, he owes this year for 2021 tax year but ther is no estate or any money to pay his bill, what do I do?

So sorry for your loss.   The final return for your son needs to be filed as normal reporting all income and eligible expenses as normal.   As for the taxes owed, if there is an estate they should be paid from the estate.  

In the event of no estate,  family and friends won't be vulnerable to IRS collections for your tax debt when you die. However, the money and/or property left to family and friends can be.  Upon the death of a taxpayer, any outstanding tax liability must be paid before your assets are allocated to your heirs.   If there is an IRS debt after death with no estate, the IRS will take all possible measures to collect that debt before it expires. Expect the IRS to contact beneficiaries of the Will to try to recoup tax debts.  This often includes children and siblings, and it is also common for both close and distant relatives to hear from the IRS when a debt is pursued. When settling debts on behalf of someone who has passed away, the IRS should be considered the primary creditor above other creditors.

Here are two articles that talk about the final returns for a taxpayer that may answer questions you have.  Topic 356 - Decedent and Death in the Family.