Asked to change son's ESA to UC

Hi, confused and distressed about what to do. My (late-twenties) son with multiple difficulties mostly autism based has 2 months ago moved into a care provision which requires rent (house owned by care provider, 24hrs care). Transition is difficult enough as it is. We still manage his money until a future time when provider/social worker takes over duty. When he moved out social worker said we not need worry about rent, all would be arranged. Now provider said had no rent, we find out not been even discussed let alone arranged. Further, social worker says cos son gets (income related) ESA (plus PIP) he needs changing to Universal Credit as no rent paid if ESA. We said from start he gets ESA. And has asked us to ring up 'to swap him to UC'. On my limited knowledge, this worries me sick. Cannot see why he has to forego a clear entitlement - or do we have to take it on chin - or are we worrying without cause? And does he then have to apply JSA and get work expectation despite clear ESA decision he's not able to work. Also even if was swapped, would arrears before UC get paid, and would he be without either ESA or UC even,  for a period while change processed? I thought I heard of council responsibility to pay rent in such cases, but may be wrong. We were so much looking forward to a positive transition so his future is sorted as we get older, but this aspect, and its unknown pressures, are worrying us to extreme.