Assuming your son is a senior now, his 2015 tax return will be used for both the 2016-2017 FAFSA and CSS, and the 2017-2018 FAFSA and CSS. Therefore, generating capital gains in 2015 will be counted in an additional financial aid cycle.
One idea would be to liquidate zero by December 31, 2015, and then liquidate some or all in 2016 and put the proceeds in his 529 before the day the FAFSA is submitted in early 2016.
Also note that unearned income (capital gain) is taxed at his parent’s rate. Without more information (basis, FMV, parent’s marginal rate, child’s earned income) it is unknown whether there is much of any benefit to split the sale between two years and have the FMV of the unsold UTMA assessed 25%, versus 5.6% after the allowance.
All these financial gymnastics may have no effect on receiving need-based aid. Many missing pieces - would the student be Pell-eligible? Is either parent earning above $50k? Other kids in college? FAFSA only school?