How to complete tax forms for my 15 year old son?

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 might also want to use free software from the IRS Free File versions:

https://apps.irs.gov/app/freeFile/

You do not enter a dependent's earned income on your tax return.  He can file his own tax return if he had taxes withheld from wages reported on W-2 to get a refund of the taxes withheld, but he is not required to do so if this was his only income.  His income is below the minimum filing requirements.

However, if he has income from self-employment and the income is $400 or more then he required to file a tax return for the self-employment income, reporting that income on a Schedule C which is included as part of a federal return.