You mention that your son claims lower rate DLA so wouldn’t currently qualify as a ‘disabled person’ for a Vulnerable Person’s Trust. He would need to be claiming middle or higher rate care component of DLA or higher rate mobility component to fall within the legal definition of a ‘disabled person’.
However, if a Vulnerable Person’s Trust is not an option for you then a Discretionary Trust would still be a suitable way for making financial provision for him and protecting his means-tested benefits. The tax rules are different though and they need to be carefully considered when setting up a trust.
As SarahL has suggested, I would also recommend attending one of the Wills and Trusts seminars as they go into detail about the types of Trust that can be used, the conditions for qualifying for a Vulnerable Person’s Trust and the tax consequences of each type. The Wills and Trusts team are also very helpful and can assist if you have additional questions.