Parent alienation

[Tes004]

Looking for advice/support/help
bit of background - divorced 8 years ago - 3 kids
remarried for 4 years.
kids always lived with me- ex took me to court for full custody on the basis that he was now settled with new gf and should be able to have the chance to parent full time. There has been some safe guard issues re dad and court decided on a live with order to stay with me and spend time with him.
fast forwaRd a couple of years and kids had been kicked out of his home on 3 occasions. Decided he wasn’t having them anymore since Oct last year and then out of the blue contacts them directly and asks them to stay over again. Kids choose to go - aged 16.16and 11.
but he only returns the eldest. This was in Feb - my ds started off just saying he wanted to spend more time with dad, now he’s completely shut us out - sends me some messages of how I’ve lied and manipulates his dad and gf (yes he’s 11 and not his words) I had hoped it would run it’s course and he would miss me and his family and that the realism that he would be alone a lot of the time he would come home.
but now in April and it’s like he hates me-
mug I go back to court an order no doubt will be passed with him choosing his dad as the new home because that’s what he wants. I’m so heartbroken, every time he was ment to have time with us they invite his mates over or arrange trips etc- I’ve seen him about 4 hours in 8 weeks.
he’s been acting awful at school but when he gets back home no discipline- given £20 to go out for the whole day.
I message him to say I love him and get nothing in return - I honestly can say nothing happened prior to him going there, he left with a big hug and a kiss, now every night I walk past his bedroom without him ..
his dad is very confrontational, I stick to simple communication to avoid upset but every message he responds is awful. I had a gp notification for ds today and asked why he was at gp,, was told it had nothing to do with me …
has anyone got any words of wisdom or ideas what I can do.
for the record I do believe this is to do with money . He had been there a matter of days before he applied for his child benefit.

Gp- you can call the GP and request the notes. Gp should do it for free.
School meeting- have you been in to speak to the head of heart or safeguarding lead to share your concerns and what is going on in their life? Get it on file.
16 year old- are they still going? Do they still have contact with 11 year old?
I'm so sorry, I have no experience with this, but couldn't just read and not say something. If I am honest it hit me hard and can only imagine what you are going through, I really feel for you. If I am honest I predict this to be my future.

@BookArt55 thank you for replying -
mots so difficult just running it over and over to come up with no solutions.
they go for dinner once every 2 weeks - not allowed to stay as apparently causes too much upset. I had hoped they would not put themselves through it now - but I was always cautious to let them decide . I think because they see and hear and speak up for themselves they are not wanted there.
they all had a great relationship, little spats as siblings do but would always play together. He’s not bothered about them also. Which is so heartbreaking.
if I’m honest I saw it coming, even tho he was 3 when ex left, he has always wanted more time with him. When he has been physically hurt by him, would make excuses for it.
school aware but says can’t help only offer support.
court order seems useless as police don’t enforce civil orders and so I would need to go back to court for enforcement order, which would end up in a new order and I guess due to his age would put him there as his choice … so unbelievably sad that ppl can do this