@SmileWithStyle Your son isn’t misbehaving, he’s hurting. He’s old enough to understand that his dad is taking holidays with the new family and leaving him behind, and that’s a painful thing for a child to process.
Your eldest isn’t having tantrums because he’s “bad”. He’s having tantrums because he’s hurt, excluded, confused, displaced, jealous (in the healthy, attachment‑based sense), insecure, grieving the loss of the family he used to have. Kids don’t have the language for “I feel abandoned and unimportant" they say it by shouting, crying, refusing, melting down. Your sons behaviour is communication and Dad is too dense to see it. Your eldest is old enough to understand exactly what’s happening, and it’s breaking his heart whilst the youngest either doesn't remember the Dad relationship pre separation as much or he simply isn't cognitive enough to realise what is going on.
If you tell him his Dad that “Your sons feel abandoned,” he will hear “You’re a bad father,” and he will react with defensiveness, anger, minimising, blaming you, punishing the boys emotionally so don't walk into that.
You don’t need to confront your ex directly, especially given the DV history. You can simply pass on the information neutrally: “DS1 has been upset because he didn’t realise you were going away. He said he misses spending time with you.” That’s not criticism - it’s just relaying what your son has expressed.
Your boys clearly feel secure with you, which is why they’re opening up. Their dad may not realise how his choices are landing, but you can advocate for your sons without putting yourself in a vulnerable position.
I would probably message their dad and say something like “ I've had time to consider the situation as you asked and I just wanted to pass something on because DS1 has been a bit unsettled recently. He said he didn’t realise you were going away and that he misses spending time with you. I’m not criticising - just letting you know what he’s expressed so you’re aware, because he finds it hard to say these things directly.”
And if you want to include the Ireland incident:
“After the video call yesterday, both boys were a bit upset because they didn’t know you were going to Ireland. I reassured them, but I thought it might help for you to know how they’re feeling.”