I work in a convenience store, and one of my regulars has me infuriated.
He used to have a kitten (close to a year old) that had access to the outdoors. One night he told me that she'd gone out but hadn't returned. I asked if she was spayed and he said she wasn't. I told him that female cats can have kittens when they're still kittens themselves, and advised him to have her spayed, and told him that the vet clinic a little way down the road from the shop did them for a reduced rate for those on benefits. 
He told me he'd get it done soon. But it didn't happen because he kept putting it off. Then one night he complained because she peed in various places around the house and was noisy. I told him she was in heat and if she got spayed, that behaviour would stop.
A month or so ago, the customer told me he'd given the kitten to a woman he knew, who had a cat already. When he mentioned the cat was a male, I asked if he was neutered, and he said yes.

A couple of weeks ago, the same customer asked my advice on the kitten milk we sell (Whiskas brand). I read the bottle and told him it's really only to be given as a treat and not main food. He had pouches of cat food in his hand and when I casually asked him about it, he said it was for the kitten (he feeds a stray). So I advised him that kittens need kitten food and advised him where he can buy it. But he's still buying adult food. 

I'm so tempted to tell him that kittens need kitten food because it has the meets a kittens energy requirements as well as other nutrients to help it grow and develop properly.

But I'm 99% certain he'll ignore my advice again. So next time he asks advice about the kitten, I'm also tempted to say "I'm sorry, but I've given you advice in the past and you've ignored it."

What would you folks do?

I think that is a fair comment to make. It's not like he is particularly attuned to the advice you've already given him. And you did point out how to go about accessing your advice in a way that was proactive instead of a comment only. It's on him to take care of what to do with the cats and it is also his choice to follow your advice or not. In stating that you don't feel comfortable giving him more advice when it isn't going to fruition is a healthy boundary for you in this "relationship." You care about the cats and he doesn't seem to care too much about them. Unfortunately, in setting this boundary, it doesn't mean he may be more likely to adhere, but there isn't a whole lot you can do in the situation, otherwise. If he were a friend and you were often at his place of residence with the cats, then maybe you could do something. But because he is a regular customer only, you can only do so much to sway him to take your advice. Maybe in letting him know that you don't want to give any more advice to him will change how he interprets your advice or maybe make him more aware that your advice is sound in judgement? But that is wishful thinking for the best.....

Let us know how all turns out, whatever you decide to do. 

while "we" might know what is best 
your customer doesnt and thats ok
in the grand scheme of things cat food is cat food 

the kitten is probably just happy to be eating something at this point and that is what is really important.