My son is 21 years old. last year, he earned $17000 from summer job. should i include him in my tax return or better to have him file separately?

You do not  add your son's income to your own tax return.   Was he a full-time student in 2020?   If he was a full-time student he can still be claimed as a dependent on your tax return.   If he was NOT a full-time student he cannot be claimed as anyone else's dependent.

In either case---he can file his own tax return to seek a refund of tax withheld from his paychecks.  His income does not go on your tax return.

IRS interview to help determine who can be claimed:

https://www.irs.gov/help/ita/who-can-i-claim-as-a-dependent

MY DEPENDENT HAD A JOB

If your dependent has a W-2 for his after-school job, summer job, etc. you do not include the information on your own return. You can still claim your child as a dependent on your own return.  He/she can file his own return for a refund of some of his withheld wages (he won’t get back anything for Social Security or Medicare), but MUST indicate on it that he can be claimed as a dependent on someone else’s return.  (Supervise this closely or prepare it for him!)

If your dependent’s earnings were over $400 and were reported on a 1099Misc or 1099NEC then he must file a return and pay self-employment tax for Social Security and Medicare.  You may want to use this version of TT for that:

https://ttlc.intuit.com/questions/1900583-what-is-turbotax-free-file-program