Can I claim the student loan interest, if I am the co-borrower on my son's student loans and I make the monthly loan payments (he is a senior in college)?

If you signed or co-signed you can deduct the interest you paid in 2020.

STUDENT LOAN INTEREST

Only the person whose name is on the student loan and who is legally obligated to pay the loan can deduct the student loan interest.  If you did not sign or co-sign for the loan you cannot deduct the interest.

You cannot deduct student loan interest if you are being claimed as someone else’s dependent, or if you are filing as married filing separately.

The student loan interest deduction can reduce your taxable income by up to $2500

There is a phaseout for the Student loan interest deduction, which means the amount you can deduct gets reduced when your modified adjusted gross income hits certain income levels and is even eliminated at certain income levels -  

•If your filing status is single, head of household, or qualifying widow(er), then the phaseout begins at $65,000 until $80,000, after which the deduction is eliminated entirely.

•If your filing status is married filing joint, then the phaseout beings at  $130,000 until $160,000, after which the deduction is eliminated entirely.

 Enter the interest you paid for your student loan by going to Federal>Deductions and Credits>Education>Student Loan Interest Paid in 2020 (Form 1098E)

Look on your 2020 Schedule 1 line 20 to see your student loan interest deduction

Yes, if you are a co-signer, and your child was your tax dependent in the year when the loan was made.

Further, you can continue to deduct the interest, if you pay it, for as long as the loan exists, even when your son is no longer your dependent, as long as he was your dependent when the loan was made.

And one more thing.  If at some time it is more financially useful for your son to claim the loan interest on his return (such as your deduction is limited by your income), this is also allowed.  You can treat the situation as if you made a money gift to him and he made the loan payments.  (This is a special rule for student loans that does not apply to most other tax situations.)