You have got to understand that many of the laws are a patchwork and 529s are new and do not necessarily fit prior concepts and patchworks. The 529 at the federal level represents a very unique separation of dominion and control over an asset which is the asset of the beneficiary and not of the plan owner. Because of Constitutional issues the states are free to treat the property issues differently, and some do. And in some cases there simply may not be a clear answer at all, never mind that some areas are essentilly not going to get reviewed. And in some cases even if something is different it may be moot---for example, even if under state law a transfer to ones self in context of 529 could be viewed as a gift, if the state imposes no relevant gift tax the impact may be moot at state level.
The transfer by the executor of $150,000 directly to a minor may well have posed some technical glitches: but for now the end result is good news and lets not invite visiting past glitches.
Technically I suspect a UTMA poses some issues as a UTMA is very likley in your state designed to address transfers TO a minor and not transfers by minor of his own funds. But I suspect this point will never come up as an enforcement issue.
Essentially as a matter of law we are dealing with is a step prohibited--if its not a prohibited action then why not. (And in some cases a step could be prohibited but carry a zero penality and you need to read betwen the lines on that one. One specific relevant example : in a state which prohibits anyone BUT the plan owner from contributing to a 529 for a non owner to contribute could pose some glitches --be it well intentioned cousin, friend, or beneficiary--but one needs to think thru if there is a real penality or even any enforcement and ways to side step it--from what I read the penality consists of returning the money to the person making the transfer--now no plan designed to attract funds is going to try to find way to reject funds. Now even if that prospect existed I'll bet its obfuscated by use of bank check.
So much for the ramblings, lets narrow it down.
In context of the specific state law for the 529 you have or seek to have is a contribution by beneficiary to the account a prohibited transaction or not. Hint, its NOT prohibited in something over 47 states.
Does your state impose a gift tax and does your state define a transfer of funds to ones self as a gift (that should be an obvious oxymoron but hey, state law's differ.)
Show me where under Federal law to transfer ones own funds from one of ones own accounts to another of any account set up for me as beneficairy is a gift. Hint, it will be a long search! That said, once the funds are in the 529 then there are clear specific gift rules for any subsequent changes.
If by some chance you were mistaken as to 150,000 being in childs account and meant it was in a custodial account for him, repost. There are at least two views on how to address same, I'll give you two if necessary
Personally I suspect you will run into no real world legal problems with either to use your rexisting 529 or to set up a new one with a UTMA.
As a practical matter a 529 you hold is far more flexible and there are some other restrictions as to UTMA/529s
A 529 is NOT a tax deferal tool--it is a tax ELIMINATION tool if properly used for education, a better tool!
If college aid is a consideration, to hold the money in his checking account is a disaster . Under current rules a UTMA/529 doesn't count-BUT this rule changes in about a year--so that a 529 held by parent or a UTMA/529 count the SAME and in both cases it is as if they were held by parent. There is one further wrinkle you didn't mention, if sons funds are parked in a 529 owned by somebody other than child or parent then they are completely out of the asset picture under FAFSA. Now this poses other risks and only makes sense in some very narrow fact patterns. But if you think it matters in your case, run thru some financial aid models on a site such as finaid.org