Sales of stocks, mutual funds, and other investments are reported on Schedule D. Since you state that he has held them for greater than one year, the information from the 1099-B would be reported in Part II of the form, long-term capital gains and losses.
The amount in Box 2 represents the sales price of your son's mutual fund. It does not represent additional income per se, but does need to be reported on his tax return. This amount is reported in Box 8d of Schedule D. The cost basis of the mutual fund is reported on line 8e (this should be in Box 3 of the 1099-B). The only amount that is taxable (or deductible in the case of a loss) is the difference between the selling price and the cost basis.