A few key points:
(1) Don't try to justify to yourself not reporting income to the CRA. If you have an obligation, file it, whether or not you expect to be caught for failure to file.
(2) If the bank issues a T5 to you, they also issue one to the CRA. This isn't an issue of the CRA 'looking into your bank account', the bank has an obligation to provide income slips to both you and the CRA simultaneously. Failure to report amounts that the CRA has on file is the most surefire way to get audited.
(3) If your brother's only income is $500, he will not owe taxes in the year - the current non-refundable tax credit [the amount you can earn each year tax free] is above $10k.
(4) Your brother does not need to file a tax return, from the information you've presented, based on the CRA's official stance: https://www.canada.ca/en/revenue-agency/services/tax/individuals/topics/about-your-tax-return/you-have-file-a-return.html - [basically, he owes no taxes, among other things]. However if this were $500 from employment income, he should report anyway, because he will earn RRSP contribution room based on his earned income.