Prune juice

So my almost 12 week old has a bowel movement once a day which is great but it literally takes him around 12 hours of struggling, crying, pushing to get it out. It’s mushy so it’s not like he’s constipated. I emailed his gastro who said to give him 1/2 oz prune juice 2 times a day with his formula. I was reading that juice isn’t good for kids younger than 6 months. I know I should probably trust the gastro but I can’t help but feel nervous. Also not sure if I’m over reacting to him needing any intervention because it’s not like he’s constipated.