Oh Bunglie, what planet are these people on?
ON a technical point, was your ds's Trust Fund a bare/Nominee Trust - basically just a savings account set up as being for (you) with regard to (your ds) - or was it a legally drafted trust?
If the latter then there are probably clauses in the trust document about what a trustee can and cannot do, and about who can benefit from the trust. I'd be very surprised if the terms allowed you to do things with the money other than pass it to your ds, and there may well have been a clause specifiying that the trust would wind up on a certain date (possibly his 18th, 21st or 25th Birthday) at which point it would pass to him.
If it was a bare trust then, I think (this isn't an area of law I know very well) that it would automatically wind up on his 18th birthday and the assets transfer to your ds. So there would be nothing (legal) the AP's could do to stop your ds having the money.
Either way were you to do what the AF seems to be suggested and use the money for yourself the Inland Revenue would be very interested, as for all the time it has existed income in a trust has been considered to be your ds's not yours, so you cannot just say now "no it's mine".