Damn ex, Stop ignoring our son!!

Sorry need to do this before I find him.

Long story short, oldests dad took off after I left him after wasting 4 years on him. Hasn't bothered providing or bonding with son (I had to beg him to sell his Xbox to buy milk and nappies as I had nothing and had just paid a £460 debt of his!!!)

Anyway he hasn't seen son in over 2 years or made any contact in the same amount of time. His choice, I told him he could Skype with son, write letters etc he hasn't bothered, instead he goes around spreading lies, saying he's messaging me every week, I kidnapped my son, my fiancé abuses him (he doesn't, he loves my oldest just the same as my youngest who is his)

Son asked 2 weeks ago if he could see his dad. I messaged his dad, he read it an hour or so later. 11 days later, still no response. Yet, he could be on 5 days ago to announce he's in a new relationship with the 7th person this year.

Blatant ignorance! Now I have a very emotional 5yo telling me he hates me for taking him from his dad!

I don't know wtf to do anymore