Hi Cassie
We cannot give you an answer as it would depend on the policy of your local County Council,
We cannot say for certain how much his UC will be as we do not have all the facts. I would assume that he would stay on HB as he is in supported accommodation, I assume that he is in the support group of ESA from your post, in which case if your son is over 25 he would get UC SSA £400.14 + LCWRA £423.27 = £823.41pm.
Most ESA support claimants who do not receive any disability premiums with their ESA will be better off claiming UC.
Gary
Nothing on this board constitutes legal advice - always consult a professional about specific problems