What do I do? What would you do?

This is a touchy subject that I feel has come to a point that I actually desperately need other people's advice on.

I have a 4 year old son. His dad and I do close to 50/50 custody and it works well for everyone involved.... Or so I believed.

The past few weeks though my son has said little things here and there that have me worried. At first it was "daddy smacks my bum" and when asked if just once he says "daddy smacks my bum lots and lots" and when he was in the bath he was complaining his bottom was sore one night because "daddies smacking lots". He at this point had some small bruises on his bottom that I put down to being a 4 year old. Now I am not so sure.

When brought up with his dad his dad swears he never smacks him, but has said he has threatened. When I spoke to my son and his dad together my son stressed out a bit and said no when we asked if daddy smacked him.

Then just the other day he told my partner and I "I don't like it when daddy touches me" I thought this was a really weird thing to say. When asked why he said that he hurts him. I asked in what way. He grabbed his arm and sort of flicked it. I showed on my partners arm me flicking and he said that that is what his dad does and he also smacks his bum.

Daycare has noticed a change in his behaviour to the point they mentioned that he is upset when his dad comes to pick him up but very excited when my partner, I or his nanna (dads mum) picks him up. He also cries when we tell him he is going to his dad's. His dad lives with his mum and dad.....

I guess I don't even know what I am asking for, but I don't know what to do. It's hard to not jump to conclusions, and I hate seeing my son so upset about seeing his dad. It's very heartbreaking.

Please help us.....