Let me clear up the SDP issue as there has been some misunderstanding
You can get a SDP in with your calculations for housing benefit providing you fit the criteria. You can also get SDP in with your IR ESA providing you fit the criteria.
When you phoned DWP this morning did you ask them about HB and SDP in which case their answer would be correct. The DWP are also correct in stating that your son should not have been allowed to claim UC if he was in receipt of SDP with his HB.
In your first post you stated that your son was living with his brother but then moved into his own accomodation on his own and as BIS mentioned below, your son would be entitled to IR ESA SDP when he moved into his own place.
Do not mix up HB SDP and IR ESA SDP they are different although the amounts are the same, with HB SDP you do not get any physical money, it is just an additional premium used to work out your entitlement which can be useful if you have an additional income such as an occupational pension.
Gary
Nothing on this board constitutes legal advice - always consult a professional about specific problems
