Is it okay to mix...?

It's nice to hear of a good rescue. He,s trying to wean himself. I wouldn't let him have dry food yet. His teeth haven't developed enough for for it. You can soak some in the replacer. Also about replacer- it has a limited shelf life and has to be refridgerated. I've done some rescue pup before and they have a tend to go to the food more-so than the milk. My best guess is the milk is like instant milk is to us. I think you're doing a good job and good luck.