D.D.
Are you using a regular toilet to train him or a small "potty" for kids? Boys find the idea of standing up to pee fun and usually have no problems with using a regular toilet. When it comes to defecating, the feeling of sitting on a big toilet with the open space below can be scary. Try getting some toilet seat covers and use those without tearing out the hole. If they feel the paper right below them, it may make him more comfortable going. Also, if there is a timeframe that he usually goes, take him in there and read him a book or use another form of entertainment until he goes. Hope it helps!