Please just use the potty!

I'm reaching out to you, my fellow parents, for advice because I've run out of ideas. My almost 3 year old son refuses to willingly do his BMs on the toilet. He holds is poo in and let's some out in little tiny bits. When he does this he hides. I had him just getting comfortable and using the potty for both 1s and 2s until our 2nd child was born in December. I've tried everything to get him back on the toilet but no luck. Thoughts? I'm really getting sick of skid marked pull ups and having him be scared to poop.