If your temps are constant and correct (with or without a NTD), I would be more inclined to blame either lack of exercise, or possibly dehydration long before temps. But it is always a good thing to monitor.
If I may recommend, I would start keeping a journal of feeding/defecating/shedding on him. And a good rule of thumb is for every 3 meals 1 bowel movement. By keeping records you will be able to more accurately know if he is really constipated or not. Mine go much more frequent than that, but it is a simple guide line to follow.
Try making a jungle gym of sorts out of PVC to give him weekly exercise on during the day. That will go a long way in facilitating normal GI functioning.
Check out the link I provided for you. Stan Chiras has done a great article on ETB and husbandry of them.
http://www.amazonbasin.com/article.htm
I hope this helps and was not redundant material for you.