Vets bill

My son’s been on holiday. While he was away we were looking after his three cats. Going in each day to check on them. Yesterday one of the cats appeared to be very poorly. I checked with my son and said I would take him to the vets. They do not have pet insurance, and no savings. Their finances are very much live for the moment and it appears they have things for essentials but nothing more. My son’s husband is self employed. Sadly the cat was very poorly and had to be put to sleep. I have discussed with my son about the vet bill which is likely to be around £2500, said that I would pay and we would sort something out. It’s the ‘something’ that im not sure what to do. Thinking ahead he will inherit things anyway. I still want to make sure they get some sort of pet insurance and think about the consequences of owning an animal. I usually give him £1,000 a year on his birthday, do you think it’s reasonable to stop this for two years? I can’t think of anything else other than to write it off or ask for a small monthly repayment. I want to show compassion but also want to get him to acknowledge the responsibility. Thanks for your help.