Hi.
On what basis are they suggesting he be sectioned? What you've outlined is obviously a severe case of OCD but I'm not sure what life-threatening risk he is posing to himself or anyone else by struggling even to this extent with cleanliness, unless he is suicidal because of it all.
I am autistic and have very severe OCD as well. I've never spent 4-5 hours on a single use of the toilet (if that's what you mean by "bathroom session" and all the toilet roll used) but I do spend hours a day washing my hands and forearms. The ritual I do after putting out rubbish takes over an hour even though I wear disposable gloves for it, and that's if it goes right and it often doesn't. It can take two. I also can't store food or drink in my home, or drink tap water, or eat or drink anything that needs heating or the use of cutlery, crockery, etc, so I only eat and drink a limited range of items that can be consumed straight from the packaging without being touched and I only leave the flat to buy this every other day at most. I eat and drink standing in the middle of a room and have a lot of rituals around cleaning the packaging, as well as how I open it and lift it up to my mouth so I can make sure it doesn't accidentally touch the walls or whatever before I consume the contents. This (and my hand washing) comes with unique referencing phrases and/or numbers I make up for each occasion, so I have the reference code alongside the memory of performing the ritual and so afterwards can be as certain as possible that I did do everything correctly each time. I have had something to eat and drink this evening but the last time I had anything was Sunday, so three days with no food or fluids. This is not uncommon. My eating and drinking has been some version of this for 20 or so years but the OCD goes back to young childhood (I am 43). There's a whole load of other stuff to do with my eating and especially my OCD, which covers a lot of themes, but I'm just telling you those couple of things in brief (yes, the above descriptions really are brief compared to what they could be!!) to show that maybe I am similar to your son if not exactly the same.
I've lived in a supported independent living scheme for 16 years and haven't worked for 19. No one has ever suggested I be sectioned even though I supposedly have a separate psychotic disorder as well as the severe case of OCD. The OCD has obviously damaged to my physical health as well due to the restricted intake of food and fluids, so I am at risk of harm. I was accepted by an NHS highly specialised OCD ward a few years ago but declined the place when I reached the top of the waiting list. This year I was referred to an NHS highly specialised outpatient service and will start therapy there in January, although the letter telling me I had been accepted said that someone with my particular symptoms at this level of severity would benefit more from being an inpatient and that the outpatient appointments were being offered as a "trial", which worries me because I really wouldn't be able to eat and drink or sleep or even sit on an upholstered chair in a hospital or any public place, so if they end up telling me they will only continue therapy with me as an inpatient I will have to stop the treatment. Unless they decided to section me, but it really is uncommon in patients with OCD as the primary illness, which means the staff on a general acute ward would probably not be the best to help your son or anyone with severe OCD. Are they saying why they want to section him, and what they propose to do in terms of treatment in hospital? I really would be amazed if they could offer him the specialist OCD therapy he clearly needs.
Has anyone ever suggested treatment at one of the national centres? They are mostly in London but take patients from all over the country. There is a medical ward in one London hospital trust, a residential unit and its sister outpatient clinic in another, and outpatient clinics in Oxford and Hertfordshire. NHS England can also commission inpatient treatment at the Priory in North London. I don't know much about the Priory or Hertfordshire NHS service, but the others are overseen by world-renowned experts in OCD and they apparently all get good outcomes for many patients. OCD-UK has good information on them, plus a forum that I sometimes use and would recommend you read if you haven't already and maybe join because there are a lot of people with experience of OCD at all kinds of levels, including to the level to which your son has it. Lots of knowledge about accessing services as well.