Stuck....

Hi there!
I hope I can do this this way..
I was wondering if anyone can give me some advice!
I have a 4 year old from a previous relationship.
His father sees him once every 3 weeks for anywhere between 2 & 6 hours spread out over sat & sun. This is his choice - not mine. Though I am not complaining that he is around so little.

A bit of background first- his father (let's call him "M") is abusive, mentally, financially, emotionally, physically, sexually. Basically a total jerk, unfortunately he is also legalled right up with a crooked cop brother. I left him 3 1/2 years ago & moved 350kms away from him (a move planned before we broke up!)

I am now happily repartnered. M is not.
Our son loves both M & my partner.
M is emotionally abusive towards our son. He tells him "if you hurt A (my partner) he will leave & I can live with you again " to which I reply, calmly & honestly "no, sweetheart, if A leaves then it will just be us here, Mummy & Daddy aren't friends anymore so he can't live here" stuff like that is said every time, immediately after access.

Just tonight I went in to check on B (son) because I heard a bang in his room. I asked him what he was doing "nothing, I'm not tired" so I was tucking him in & smelt smoke. I asked him if he was burning something & immediately checked his electric blanket (not in use while he sleeps! Always unplugged!) as I flicked his Donna up, I saw a box of matches fall out. I asked B where the came from, he told me out of his uncle's camping bag. I asked why he had them in his room "because if I burn the house down. We will have to live with Daddy" "no, buddy, we won't. We will live in a hotel with no toys or anything. Not with Daddy" "but we will if I tell the policeman it was A" "no, honey. Still in a hotel. Mummy & Daddy aren't friends, remember? " to which he said "but my dad said we would!!! " & got extremely angry & upset about it.

B loves A, most of the time he wants A over anyone else. I am beyond positive there is no abuse from that side, no reason from there to be this way. It is only the Mon/Tues/Wednesday after access that it has been an issue with B just being mean to A & on one occasion, smashed his knee with a hammer. But nothing I wasn't prepared for. (i have experience & qualifications in this area myself) B has not seen M for a week, which is why it's peaked my radar a bit...

Is there anything I can do to prevent this?
We already go to mediation regarding access, he used to have supervised visits but played his way & they won't grant me them again. I would much prefer to keep this out of court because, again, the crooked cop brother (who already told him what to say when he got off on the domestic violence charges, despite flat out admitting it to police originally) but I will if I have to.
Is there any type of service available for my son? Any tips from people in a similar situation?