NC sorry.
I have a 13 year old DS.
DS dad and I were teenage parents. During our early teens and early 20's he was abusive in every single way. During my relationship with him, he had immigration problems and I would help him out as much as I can. Anything he needed, I will help him. When he didn't have a job (because of immigration) I will give him money and let him stay with him rent free. He is on a temporary visa (?) and will be applying for a British passport this year.
When he did use to work, he would refuse to give me money for DS. He will make excuses that he has a lot of bills to pay, his in debt, blah blah. I got sick of it and applied to child maintenance where they were giving me £300.00 a month. DS dad came to my house unannounced one day and told me that as child maintained are taking his money through direct pay, he is actually paying more than £300.00. He threatened I should stop CM or else. He promised that he will pay CM directly. But he never did as you guessed.
Throughout our 10 years together. He would see DS intermittently. We were lucky if we saw him once every month. At most, DS wouldn't see his dad for 6 months at the time.
Anywhoo, two years ago, DS dad realised his wrong doings and said that he would like to build a relationship with his son. Therefore, for a good two months, he was perfect, providing for DS, taking him out, helping me pick him up from school Ona. Regular basis. However, his dad soon began to get comfortable and the last time we saw him was when he assaulted me in my house. This was two years ago. It went to court this year and he got served with probation:
Last week I received a call from an immigration officer or advisor telling me that am I willing to write a statement on DS's dad behalf and how he has been as a father, and DS's passport as he will be applying for a British passport this year and Home Office have asked information from me.
I told the officer no. That he is using my son and no that I will not allow it. The immigration advisor then asked me what was the contact like between our son and the dad, as he said that he hadn't seen DS due to the lockdown 
. I told the officer that he hasn't seen DS for two years, mainly due to the injunction and incident but that I have instructed the court that he can have contact with DS, but it has to be supervised and that he would need to be the one to initiate it.
The immigration officer asked me again that if I was willing to provide a statement and I said "no", that I will not allow him to use his son. That he has not made any type of effort to be a father to his son. The immigration officer sounded very unhappy and quickly hanged up the phone.
Looking back, I think it was a bit harsh. It is his dad after all and it wouldn't be nice if he was sent back to his home country. My friends tell me that I would be stupid if I supported him. But what do you think?