Answer:
What are the tax consequences of this loan to Joshua and to Seth?
The IRS requires that any loans must charge a minimum interest rate, and in this case, since Joshua is not charging any interest to his son, the IRS will consider the minimum interest rate as a gift and will tax it that way. Since Joshua can make gifts for $15,000 per year, if the threshold is already passed, the IRS will consider $100,000 x 4% = $4,000 as taxable gifts.
How would your answer change if Seth uses the money to invest in corporate bonds paying 8 percent annual interest?
The answer shouldn't change since Joshua is the one that can be taxed here. Seth cannot deduct any interest expense or gift tax expense either.