As far as a transfer is concerned, just do it normally. The banks will be required to make reports since the amount is more than $10,000 but there is nothing improper with giving or loaning money to a family member. And in fact, if you wanted to break up the transfer into 10 transfers of $9000 each to avoid the reporting requirements, that by itself could be illegal even though the gift itself is not. So just make the transfer normally and don't try and play strategy.
As far as taxes, this is either a gift or a loan. If it is a gift, there is no expectation of repayment. You must file a gift tax return form 709 to report the gift since it is more than $15,000. But no gift tax will be owed since you have a lifetime exclusion of $13 million of gifts, the form must be filed so the IRS can keep track of your large gifts. If this is a loan with an expectation of payment, then you must report the interest as taxable income on your tax returns. You must report the interest as taxable income even if you don't charge interest to your family member, this is called imputed interest. The tax position is that you make a loan, your relative pays interest, you report the interest as taxable income, and then you gift the interest back to the relative. But you do have to report the interest as income, using at least the applicable federal minimum interest rate.
https://www.irs.gov/applicable-federal-rates
https://turbotax.intuit.com/tax-tips/tax-payments/irs-tax-rules-for-imputed-interest/L7UbulHpC
For example, for a loan made in June 2024, with interest compounded monthly, the AFR is 5.01%. That means that for a $90,000 loan, you would charge your son $90,000 x 0.0501 ÷ 12 = $375 per month in interest. If your son pays more interest than that, you simply report it as taxable income. If your son pays less interest or no interest, you are still required to report the $375 per month as taxable income as if you had received it.
