Backsliding with potty training over and over and over...

My son was recently diagnosed with SPD. He was 4 in December. He has 3 siblings which are 2 years old. (triplets) We originally potty trained him almost a year ago when he told me he wanted to wear big boy undies. He had very few pee accidents and caught on very quickly. Pooping took much, much longer.

After a ton of patience and some corn syrup we successfully had him pooping on the potty after about a month. Over the next six months he would occasionally have setbacks where he would go in his underwear, but usually we were able to get him back on track within a few weeks.

However, this last bout of trouble has lasted over 2 months and seems to be getting worse. He has not pooped on the potty in over a week and is now going on the floor. My husband is beside himself and I'm out of ideas.

Is this normal? What can I do? I am having his sit on a potty sear and putting a stool under his feet, but he still won't go.

Thanks for any help.