He cannot move back in with you, and he needs to stop thinking of this as an option.
You do not have the physical space for him. You do not have the financial means to support him. In fact, you will be worse off financially from losing help you currently get, that him moving in would disqualify you from, and probably then be unable to even financially support yourselves.
He is rude and messy to the point family who DO have room, will not take him in. He will have expectations of being cared for rather than being responsible for himself.
He is a 27 year old man who needs to figure this shit out himself. Legally your responsibility to house feed and clothe him ended at 18, given he is not disabled and incapable of working or caring for himself. You are NOT obligated, legally or imo morally, to take him in.
Firstly, he can ask his dad for help. He is as much his parent as you are so has just as much "responsibility" to help him if he is genuinely able (which you are not). If he doesn't think his father is obligated, then he can't reasonably think you are either.
His room he rents, totally get its the cheapest available and he can't move to a cheaper one. As a single male under 35 with no dependents he's only entitled to his local HA's room in a shared house rate of housing benefit, which is not enough to cover full rent. Does he have a guarantor who legally has to take over his rent when he can't pay? Are all tenants in the property liable for the entire properties rent getting paid, and legally obligated to pay his share/shortfall if he doesn't? Has he applied for discretionary housing payment from his local council, who may agree to pay the difference between his rent cost and housing benefit, even if only for a few months? If none of these are applicable, what he needs to do, under his rental agreement and more importantly the renters rights act, is simply not pay the short fall in his rent. He WILL get into arrears, and only after the legal time period stated in the renters rights act, will he have the eviction process started against him. If he gets a job before this point, he can pay off his arrears and avoid eviction. If it goes on long enough that he is eventually evicted, then when bailiffs arrive to kick him out, he presents to the council as homeless. It is important he doesn't leave his housing voluntarily, (such as moving in with you or anyone else) as this will be considered intentional homelessness and void their liability to help him. The process of eviction will take at least 12 months before his is forced out of his room.
Is his phone contract fixed term where he is still paying for the handset as well as his bundle? If so nothing he can do to reduce costs, but if he is on rolling terms for his bundle and owns handset outright, he just needs to give a months notice. He can then get a cheap sim only plan, and even sell his fancy handset and get a cheap phone. If he cancels the direct debit and refuses to pay, his SIM will be shut off, his phone potentially bricked, and he will be taken to court to pay his outstanding contract amount, plus fees and incurred charges, resulting in a CCJ.
He will be getting his £400 a month new style JSA/ UC standard rate as long as he complies with job searching etc as defined in his claimant commitments. He should prioritise first any council tax he's liable for, then (if he really can't sell the car and cancel insurance) not letting his car insurance lapse, as he needs to be covered in case of an accident. Then his food, and the most basic SIM only monthly rolling contract, in a cheap basic handset.
Yes, doing these things can tank his credit score, get him CCJ's and affect his future ability to rent, get credit etc. But it takes time to get to that part in the process and will motivate him to get any job possible, to take payment plans to pay back arrears, accrued debt, etc to prevent this happening, and get back on his own two feet.
His is, under the renters rights act especially, very protected for a long time before he actually ends up homeless.