Just found this topic - I've been posting in the step- section, but it might be better here, tbh. This will be long, but it helps to set it out, I'm hoping that someone might have some advice or even words of support would help at this stage!
DSS (he's eight) has been recently diagnosed by an on-call GP with stress/anxiety that is causing physical symptoms of recurrent diarrhoea and stomach pains.
He is a very sad, withdrawn little boy, and he IS a little boy, because despite his age, he appears to me to be emotionally immature, although my experience has been with DD and I know that girls mature faster.
I'm starting to think that there are some fairly deep-seated issues with DSS, rather than a superficial problem, but I fear that he will not get any help to deal with them.
DP and his exW were married for 12 years; they had a daughter within the eighteen months of marriage and DSS came along as an "accident" six years later. DP discovered that his exW was having an affair when DSS was four years old - exW justification for the affair was that she had not been happy since she fell pregnant with DSS; she said that she felt that DP never wanted DSS because they had discussed a termination early in the pregnancy. DP filed for divorce after exW refused to go to Relate and although she wanted him to stay, she wanted to have the freedom to have affairs with other men!
The end of the marriage and first year of separation was traumatic - the DC's got caught up in the middle a lot. DP moved out of the family home and bought a new place where he could accommodate the DC's, but their mum didn't want them to have anything to do with their Dad, and exposed them to a lot of her anger that DP wasn't doing what exW wanted. DP doesn't know everything that was said, but he knows that the DC's were told things about him and the marriage that they probably weren't equipped to deal with; for instance, all the legal correspondence between DP and exW solicitor was shared with DSD who was 12 at the time. After about a year of inconsistent visits, changes at the last minute and lack of communication, DP had got a court order setting out the DSC's residency (with their mum) and a contact order for DSS (from Thursday after school to Monday morning every other week) and things settled down a bit. (No contact order was put in place for DSD due to her age).
I moved in with DP soon after the court order was put in place, and the DC's were directed by their mum not not have anything to do with me or BD as they were visiting to see DP, not us (their mum told DP that is what she had told the DC's).
DSS has been coming every other week for about 18 months, with half of all school holidays here as well. Unfortunately, his sister decided after about six months of inconsistent visits that she no longer wanted "two homes" or "two families" and she has cut off all contact with her Dad. DSS has said on occasion that he enjoys having time here away from DSD, but he has also said that he doesn't think it is fair that she can choose whether or not she comes and he has to.
Over the last 18 months, DSS seems to have experienced a rollercoaster of emotions. He and DD got on well straight away, but it (understandably) took a while for he and I to develop a bond, and we didn't push that, but after a very enjoyable family holiday, and a great time together last Christmas, I thought we had begun to develop a bond as a family of four (DP, DSS, DD and myself). He did appear to me to be very "young" for his age though - he expected help with toileting and basic personal care, he did not seek or enjoy independence, and he seemed very concerned about his mums wellbeing when he was with us, but we got to know each other and he began to show enthusiasm and enjoyment when doing things with me. However, soon after Christmas, DSS withdrew from me completely, and refused to speak to me, or even make eye contact with me, during his visits.
After some difficult conversations with DSS and his mum, DP discovered that DSS had overheard his mum and sister talking about me and BD, and he had misunderstood what they had been saying, believing that they were being mean about me. This had affected the way he viewed me and BD in his life. Despite the obvious distress it was causing, his mum resisted DP's attempts to secure some youth counselling for DSS - after a lot of hassle, DSS did eventually go along for a few sessions, and it helped rebuild his trust in me.
That was earlier this year, and we had another positive few months together over the summer, and DP's relationship with his exW was becoming less strained as well.
In the last few weeks though, it feels like we are back to square one. DSS has withdrawn from me & DD again and this time DP as well - limited conversation and resisting eye contact, further regression in terms of emotional understanding and behaviour, excessive fidgeting, appearing uncomfortable in company, lack of enthusiasm about activities that used to engage him, disturbed sleep and just "unhappy" in himself.
I realised that DSS had a stomach upset a few visits ago - he had been off school ill earlier in the week, so we thought it was just an infection, but DP discovered that DSS didn't realise that diarrhoea was abnormal and the conversation revealed that he suffered regularly, when he was at both houses. When he had another bout the following visit, DP took him to the GP, who asked for some background, noticed DSS behaviour (he picked up on the fact that DSS had infected fingers due to the extent of his nail biting, for instance) and diagnosed anxiety as the cause of the diarrhoea. He said that he would refer it to DSS usual GP who would be "in touch".
I am so worried and concerned for this little boy. I feel that he would benefit from some form of talking therapy, but realise that only his mum can facilitate that, and if she chooses not to, DSS will be left to struggle with this on his own 
It's such a shame, DP is a great Dad and would love to spend more time with his DC's - instead the limited time that DSS does have with his dad seems to be causing him conflict