Constipation in Child since 3 wks old

Hi, i need help. My son was diagnosed with a Cow Milk Protein Allergy at 1 month old. he was put on amino acid formula since then. first it was Elecare but i felt like it didn’t work for him so we switched him to Alfamino a couple months later. he’s been on alfamino for most of his life. He also was diagnosed with Reflux. I had came here before to buy NatPhos6X for his reflux and it honestly was the only thing that has ever helped him. we gave it to him at 5 months old for 3 days 3x a day. his reflux went away and came back a month later so i did two more doses and then he was fine. he occasionally still gets reflux if he has something that has milk in it. for example, alfredo pasta. he is now 15 months old almost 16 months old on the 15th of march. he still is allergic to milk. his allergy shows up as constipation and reflux. Since he was 3 weeks old (when he started his formula) he has been constipated. we’ve tried prune, which would get him to poop mushy up until 6 months. then it wasn’t working, we tried other remedies but nothing has really helped him. it turned from mushy poops (at 3wks old) to balls (around 4-5 months old) to now being little dry pebbles, skid marks on diapers without poop, or thick hard and dry turds. I’ve tried using a butt suppository and did not work so tried it again a couple hours after and he finally went to the bathroom. when he does poop now, he screams and cries while it comes out and even after it has already came out. nothing seems to calm him down except for time. there’s no blood in stools, it’s not mucousey. it just dark brown and dry or dark green and play dough kind of looking. please help! i’m in such desperate need for him to just regularly go without screaming and crying. i feel so bad for him because he has always been constipated. he still drinks formula and i’m transitioning him out of his slowly to an oat milk because he is allergic to dairy still. he eats regular solids on top of that, fruits, some veggies and only drinks water. he has a lot of water throughout the day. i’ve tried chia seeds, juices, probiotics etc and nothing seems to help him. he is a very active one year old. for the past 3 months he’s been at the same weight of 25 lbs. he hasn’t really been gaining weight or losing. it fluctuates between high 25lbs or low 25 lbs. he goes to the park for an hour almost every day. he sleeps pretty well also. we have started to completely cut dairy out so no more sprinkles of cheese, no more small amounts of yogurts. nothing that has milk in it since it seems to just be getting worse when we do give him something with minimal milk. can someone please tell me what to give him???