How To Potty Train A 6 Year Old?

Hi ,I have a 6year old ,wondering if u can help ,he soils in his pants and it been goin on for quite awhile I have taken him to doctors they given me lacatose he's had it before but that doesn't work e still kept doin it in his pants and he also had mavoicol .ive even tried with a reward chart and putting marbles in jar for when he has actually been toilet to get his poo out and for haveing cleans pants too, and I've even tried with money in a jar