Sorry this is complicated, and for the divorce. That is never easy for anyone.
It seems to me that the tax guy may be right, but the benefit in liquidating is for the future years. You have to look at this over the full 4 years - not just this one year.
The UTMA in your kids’ name becomes 25% for the first year, then again in the next 3 years after that, so taking the hit this year in taxes means a wash this year, but you have additional savings after that as for the next 3 years it counts as your asset instead of his, so it is included at the lower parent rate when included for EFC.
However, the other complicated part of this is the fact that having higher capital gains this year means your overall income is increased a LOT this year, so your EFC may go up.
Even if the kid files independently - which is questionable as he is getting support from his mom, and presumably from you as well - as I understand it, many schools will still need to see both parents tax forms.
Given the magnitude of the dollars involved, it might be worthwhile to pay for a tax advisor this year, as there may be other issues not mentioned that could also be affected by the choices you make. In my mind, I think you may need to sit down with a professional, make informed decisions, and potentially take action before 12/31.
Best of luck.