Eating small amounts will not hurt him, however cat food is very high in fat so if may cause vomiting and or diarrhea if he eats to much of it and in some cases can lead to pancreatitis.
If his appetite has increased recently I would suggest getting some senior blood work done. This could indicate underling health problems such as Diabetes, Cushing disease and problems with the thyroid.
I hope this helps, please reply back if you have additional questions or concerns
Thanks for your answer which leads to more questions :
My dog is a bitch. She doesn't eat a lot of cat food but occasionally I have let her empty a half finished dish of cat food so I will stop this now. She does not get diarrohea but sometimes misses a daily faeces.
She has had for some time a small lump on her throat which appears not to have grown (about the size of a large marrowfat pea like Batchelors). Her throat is quite fattly which I liken to human's in olld age. Would this indicate thyroid trouble ?
Other then her neck how does she look in the tummy area does she appear to be over weight?
Does she have a pot belly?
Has there been any change in her drinking or urination?
Does she seem to have had an increase in her appetite or has she always been a big eater.
She is a LITTLE bit overweight due to not being walked for as long as usual because she seems to tire more easily unless off the lead and chasing a rabbit when she somes to life. She has never liked walking on the lead !
She does not have a pot belly.
Yes she is drinking more I would say. She seldom drinks tap water in her drinking bowl in the house but certainly likes to lap up a rain puddle especially a muddy one. I would say she is urinating a bit more than usual. I have a smaller dog also 13 with whom she keeps up when off the lead..
Increased drinking and urination and either weight gain or weight loss is seen more often in diabetes and Cushings disease.
Without doing some testing their is no way to know for sure. I would get a senior exam and a senior blood panel done. This is one test that will check the thyroid , blood glucose and in most cases there are changes in this blood work that would indicate testing for Cushings is needed.
I would also have them look at that lump it may be a simple cyst but in a senior dog you need to be sure this is not a cancerous growth.The vet can remove a few cells from the lump using a small needle and look at them under the microscope. This is called a Fine Needle Aspirant or FNA.In many cases you vet will be able to tell you want it is by what types of cells are present.
A FNA can be done on the lump and no this does not require anesthesia. This is no more painful then getting blood taken or vaccines.
This will explain how this is done