Please be gentle, I'm really finding this hard.
I don't want this to be a long post so I'll get straight to it.
I do not condone or agree with smacking a child. My ex and I have been split up 4 years. On a few occasions my son (he's 9) has come home and told me his dad has smacked his bum, he's then gone on to make me "pinky promise" I won't say anything to him, so I didn't
I want my children to trust me and be able to confide in me, which he did, so I honoured his request to say nothing. Stupid I know, before anyone points that out 
Last weekend my son came home and his father has hit him across the face , I'm sick to my stomach. This time my son has asked me to stop him, which is great as finally I can do something about this. I mostly definitely would have done something regardless as this is unacceptable. His dad and I do not get on and he's not a nice person, I'm scared if I go to him with this he will take it out on our son the next time he is with him (every other weekend)
I want to report this. I've been told that it is not illegal to smack your own child in our area until next year! March to be exact!! This is disgusting and ridiculous!
Can I go to a solicitor and threaten no contact if it continues? I can't get through to child protection and was advised by recorded messages to call 999 if a child is in immediate danger. Social services will only act after the 4th report. There's so much focus on abused children, but absolutely no help or advice for children being smacked by their parents!!! Across the face!!!!!!! It's escalated!
Thank you for your time 