Good morning, I would like to help with your question today.
Your son is still very young, and not being completely potty trained at this age is completely normal. Rather than strictness or punishment to encourage your son to have his bowel movements in the potty, give him encouragement, and praise for his efforts to do so. Do continue to talk to him, and let him know that big boys and girls don't go in their pants they use the potty; but associating having a bowel movement in his pants with being "bad" could work against encouraging him, and make him try to hide or be ashamed when he does make a mistake. If h doesn't make it to the potty, don't make a big deal about it, simply remind him that big boys have to make it to the potty, and assure him that it's ok, and that he can try again next time (and maybe if he does, he can get a prize); Use rewards instead of punishments as a way to encourage your son to use the potty.
My twins aren't on the potty yet, but if your son is an Elmo and Sesame Street fan like my girls are, he should love the book, "Potty Time With Elmo," and there is also a video and an Elmo potty time doll available as well: http://www.amazon.com/s/?ie=UTF8&keywords=potty+time+elmo&tag=mh0b-20&index=aps&hvadid=21238894&ref=pd_sl_65icketd77_e