Infant dyschezia

My 4 week old is really struggling to poo and has since he was 3 days old. We thought it was his milk at first so we changed it and he started to poo once a day for 3/4 days. He then stopped again and didn’t for 4 days until last night where he managed to poo but in the bath with his legs held up. Safe to say 4 days worth left his little body 😖 He strains and strains constantly and especially through the night, he goes up to 4 days usually before he manages to go but he’s constantly straining with his legs in the air and his arms all over the place, he grunts constantly bless him. When he does eventually poo it’s always a soft normal poo, never hard or any mucus. He lets out lots of wind and doesn’t cry a lot for his milk, he can go sometimes 5/6 hours without it and I’m wondering if it’s because his stomach feels so full all of the time. He’s never sick and doesn’t show any signs of an allergy so I don’t think it’s his milk. We had him at the hospital where they told me his immune system is just immature and to basically just deal with it until he grows out of it. They refused to give me anything to help him go. Anyone who has had to go through this with their little ones- how did you help them? How long did it last? I’m a first time mam who worries constantly, hate seeing my baby in pain 😔