You do not report the capital gains in your son's UTMA account on your tax return. If it is reported at all, it goes on your son's tax return if he files one (or if you file a tax return for him). The 1099 for your son's UTMA account should have his Social Security number on it.
If your son has no earned income (from working), and his total unearned income is $1,300 or less (for 2024) he is not required to file a tax return. Unearned income includes all investment income: interest, dividends, capital gains, etc. You can use the calculator at the following link on the IRS web site to see if your son has to file a tax return.
Do I Need to File a Tax Return?