I would certainly insist on repayment. You and your son need to work out how much he can afford to pay you monthly until the debt is cleared.
Personally, I would feel very mean if I stopped giving presents to clear the debt that way, but I might well ask, when his birthday approaches ,if he would like you to take out a pet insurance for him - if you feel you can afford to do so. I have no idea what these insurance schemes cost, as I have always just paid vet's bills our of my current account.
In my younger days, I considered taking out insurance for a newc at, but as the charges were twice what I had paid during all sixteen years of my previous cat's life, I decided against it.
Sometimes paying bills as they come in is possible, other times it meant doing without something I wanted, but the cat came first.