If the son has been in touch with agencies for support, then I would say that the issue is really the lack of timely external support.
My son is very similar to the young person described here, and we have been in touch with service after service. MH services seem to be a continuous round of referral from one service to another. The social worker who was supposed to be conducting a needs assessment with my son, turned up once to introduce herself, then has never been seen again.
I would advise the YP (or the mother if this is appropriate) to continue asking for help from public services, but I am afraid there simply may not be any help available.
Are there any community cafes or voluntary work places available in the YPs area? Could he make enquiries, independently?
If he feels suicidal, there will be a mental health crisis health-line available, which the YP or his mother should call.
The weighing thing is strange. If the YP has an eating disorder, his GP needs to be involved and BEAT, the eating disorder charity can help. My son has an ED, and although BEAT have stated his needs are too complex for them at the moment, there are support groups he can access.
With respect to abuse, perhaps the mother does not know that her very protective and 'proactive' behaviour is actually preventing her son from developing essential life-skills, restricting his freedom and autonomy and (in terms of the weighing) possibly degrading her son. It could be regarded as emotional abuse (or even neglect). I feel she may instigate this control with good intention, reasoning for example, that if she does not do something, no-one will, but she needs to realise her control is not helpful.
Unfortunately, there is little that you can do for the mother. She will probably regard advice as interference. However, if the son has reached out to you, tell him to make an appointment with the GP about his mental health, if only to access the MH crisis team phone number. He could also approach Adult Social Care, who may respond better than they did with us. Yet, if there is any way, the son could access a group outside the home, I feel this would be a huge step towards his independence and the first stage of building up an informal network of support and increasing his confidence.