Trusts

We are currently updating our wills leaving everything to our two sons One of them receives income related ESA and DLA and we are planning to set up a discretionary trust for him instead of leaving him a lump sum. Does anyone have any advice on the best way to do this and any potential pitfalls?

Someone has also suggested that it is possible to set up a trust fund whilst we are still alive which he could use to supplement his benefits. Is this true?

We want to help him as much as we can (both now and after our deaths) without the danger of him losing his benefits.

Any advice or suggestions would be very welcome.
Many thanks
Tricia