am giving him puppy milk ?
You can get puppy food in the market but I home feed my puppy with rice and boneless chicken. I adopted my lab when she was 5 months old too, she is 8 months now... Also try a pumpkin sauce thing that you can but at the pet store it helps their stomach. I also give her yogurt once a week to help her fight ear infections. I don't know what puppy milk is so can't comment on that.
At 5 months your puppy should be eating dry puppy food or dry puppy food with canned puppy food.