My nephew had the same issue...

His mom took away his potty chair and placed him back in diapers as only babies pee & poop in their pants she explained.

He was OK for a short while and then decided that he no longer wanted to be a baby and wear a diaper...

Has your son given you and / or your husband some sort of explanation (that a 3 1/2 year can give) as to why he poops his pants?

Maybe by removing something that he truly loves and reward him when his does right will get him back on track?

Come to think of it...my daughter (now 20)...I did have an issue with her pooping in the toilet...she did OK for a while and then she stopped...discovered that by her pooping in the potty spooked her, to see what came out of her scared her but it was different with a diaper as she did not see it. Finally she would poop in the potty but did not want to see it. That worked out fine. Until she started to use the regular toilet, flushing absolutely terrified her...she finally outgrew that as well.

Not saying this is the same with your son but just my personal experience.

~Zig

I too have a 3.5yr old boy that has been poddy trained for months. He hasn't had any accidents at day care,but once every day for the last three days he's messed himself. He seems to be waiting untill last minute and starts holding himself and says "i can't hold it" as it starts to 'exit'. We took the tv the first night (and he hasnt got it back) b/c he said he didnt want to stop watching his show, put him to bed early last night,after a bath, and today..at mcdonald by the way, i cleaned up as best i could and made him ware them all the way home and then for a while. His responce is that I'm being mean to him. WHAT DO I DO? DID U FIGURE IT OUT?

My son didn't figure all of this out until age 5. It was a nightmare. By age 2 he stopped wetting at night. By 3.5 he used the potty to pee. But he would hold in his poop and "smudge" or just poop himself. We tried tough love, we tried stickers, other rewards, long talks, books, doctors, on and on.
December came around. This sounds awful, but we were at our wits end...we told him Santa did not bring toys to 5 yearolds who pooped thier pants. All of December he did great. We also used the method that had worked best in the past, everyday at 4pm he had to go poop. It took a few weeks for his body to regulate itself, but now he does it all on his own. After Christmas we told him, "Now we know you can poop in the potty like a big boy, no excuses" Big boys also get to stay up a tad later, but not if they did not use the potty. Anyhow, long involved I know, but it worked for us!