Legal advice

I'm after some legal ownership clarification. Can you help?

My ex and I had a dog together. When my ex cheated and left me for someone else I took the dog (my ex put up no fight). I have let my ex see the dog every 4-10 weeks since (my ex left 18 months ago). The dog was transferred into my name alone from the point that my ex left. I would now like to move on and stop all contact - my ex has a new life AND a new dog now. This is partly for me and partly for the dog - I have noticed behaviour changes in my boy after my ex's visits, OCD cleaning, angst and sulking. I know he can't tell me what's up but I know something's not right! Thankfully it only lasts a few hours and he goes back to his normal cute, funny and loving self.

Am I running the risk of losing the dog if I ask my ex to stop seeing him? The last things I want are to lose him altogether (or even part time if a dual custody is ordered), or for him to suffer in any way at all.

Please help if you can. Many thanks.

Anon