Constipation, 5 week old (help)

My little man is a little over five weeks and the past three weeks he has been dealing with constipation. He is going to the bathroom once every 1-2 days, however, his stool is very hard and you can tell he’s uncomfortable and in pain. I’ve been working with his pediatrician and we’ve had no real solution.

Temporarily- she had him take prune juice; initially 1tsp daily. Then bumped up to 1/2 oz 2x daily. It took a few days but the prune juice started to work. After 5 days his stool was a perfect consistency.

Formula: started at hospital with Similac Advanced Pro; switched to the Sensitive belly one... a week and a half ago, pediatrician recommended Soy (so he’s been on prosobee).

Doctor had us stop prune juice 2 days ago to see if the soy formula made a difference - he’s back to hard stool (pellets).

Please share your experiences and what has worked and hasn’t. I’m concerned and feel bad for my little one.