pediatric gastro problems

My son is 29 months and when he was a few weeks old was diagnosed with mspi. We did the special formula and he always had pain and a hard time passing stools. We did soy milk until 18months then switched to regular milk with no new problems. His stool is like playdoh not hard but not soft, has a unreal smell to it, he has a very hard time going sometimes it wil take him days to get it out. We tried miralax, caster oil,have eliminated certain foods and have had to do enemas. We met with a ped. gastro specialist who did an allergy blood test and came back with no none allergies. So now he did ok for 3 months this summer with no change in diet or medications. He is curently not taking anything and eats a balance diet but the contsipation has gotten way worse and is going 5-7 days before he can go and we dont have any diagnosis for whats wrong with him or suggestions as to what to do. All they said we can do now is biopsy his intestines but I'm aprehensive any info would be helpful

[CHELLETO6DOGS]

has anyone thought that he may have polyps (not sure of the spelling) my son had them when he was a few months old. though he didn't have that color stool my youngest had the color stool and the smell.his doctor didn't like that fact and sent him over for a sweat test, to check for CF, thankfully it was negative. does he have any other issues you may not think it could be related?
good luck
Michelle