[Solved] Ex wants to take my son on holiday during termtime

My son is nine years old and in Year 5. My ex wants to take my son away on holiday to Florida in June during term time. She previously took him to Florida in November 2016 and he missed almost three weeks of school. I had originally thought she was taking him during the half term, but going either side of the break to minimise the amount of time he would have off school. She was happy to let me believe that until about a month before the holiday when she corrected me and said that the school were OK with it, due to my son having had near perfect attendance throughout school. This turned out to be false and she was fined as the school and local authority have a zero tolerance policy to absenteeism.

I've told her that I would prefer that he went during the school holidays, or either side of the holidays to reduce the time away from school. I appreciate that much of the curriculum is complete by mid June and don't think this holiday will be as detrimental to my son's education as the previous one, so I've outlined my position which is "I'd prefer that he didn't miss school, but i wont stand in your way".

She's now asking me for a letter stating that I support her decision to take him away and give my permission, because apparently US customs are more vigilant on single parents entering the country with children since Trump came into office. My understanding is that she doesn't need my permission to take him out of the country, unless it's for a period of more than a month. He'll be gone for 10 days. I'm reluctant to say that I support her decision, when I don't.

Do I have to do this? Things are rocky between us at the moment.

A bit of background. Since early 2016 my ex has been involved in a new relationship, which has caused some distress to my son and his behaviour at school and her house took something of a nose dive. His behaviour with me has always been fine, but his academic attainment also suffered during this time. Upon his return from holiday in 2016, I worked patiently alongside the school to bring his grades up and he is now age related in all areas of the curriculum. His behaviour at school has taken longer to address and we are working through an Individual Behaviour Plan (IBP). In addition he has started a programme of art therapy. None of these behaviour issues were present until my ex embarked on her new relationship. Over time I became aware that her new partner spent time in prison for violent robbery and his children are subject to a child protection order and he has a supervision order for them, as the children were removed from the care of the mother (she has substance abuse issues and is believed to be a prostitute). He has custody of five children, but the eldest two are not biologically his. He was awarded PR, but subsequently tried to put them in care and they temporarily went to live with their biological grandfather. He let them see their mother and created a safeguarding emergency that resulted with the two children being returned to my ex's partner. I believe the deterioration of my son's behaviour is linked to everything that has been going on at his mum's house and I have sent a solicitor's letter to her asking her to attend family mediation in an attempt to avoid involving the courts, pending the advice we receive from the art therapist currently working with my son. My son has expressed a desire to live with me, but I'm keen to resolve the issues he's experiencing and maintain the shared residence arrangement we've had in place for eight years. I think it's important that he has balance. I have him half of the time but this arrangement isn't enshrined in a court order, If we can't resolve the situation for the betterment of my son, I'm prepared to apply for residency.