Thanks for your query.
is basically a nutritional issue unless some pathology is there in the large intestines. As you have mentioned his diet which is very low in roughage he is bound to be constipated and there has been no effect of the laxatives given to him. I suggest you modify his diet to include roughage. You can visit the site http://www.vardaan.net/hl-chfd.php where I have provided details for such a diet.
Simultaneously you can use glycerine suppository for a few days so as to facilitate the removal of hard stools till the diet starts having its effect.
I hope I have answered your query to your satisfaction.
Wishing your son all the best..