Filing for son

The minimum income requiring a dependent to file a federal tax return. 2022 filing requirements for dependents under 65: Earned income of at least $12,950, or unearned income (like from investments or trusts) of at least $1,150.

Self employment income over $400 is subject to self employment tax.

What about interest income (he cashed in some bonds)? Is that considered part of the $1150?

Yes, both interest and capital gains are unearned income.