The photos were a bit small to see any detail, but from what I could see, there wasn't anything glaringly obvious that would cause the constipation problem you are having, given that your lighting and temps seem ok.
Are those all live plants in there? A bite of a plastic one could easily lead to impaction. But even live plants sometimes have hard leaves that are more difficult to digest (think bran in human terms), and this could cause some blockage.
Also, you may not have seen him eat any soil recently, but it just takes one stone to get lodged in the intestinal tract to cause impaction.
You did say you saw one dropping in the food bowl. Do you know when that was from?
If he goes for more than 4-5 days without leaving any droppings, I would take him to a vet and have him examined. It's really not normal for them not to be regular.
If he is in fact leaving droppings, but irregularly, or his droppings are looking dry and without urates, he could be experiencing a genuine constipation problem caused by his diet and/or a lack of hydration.
Try increasing his watering a bit, and feeding him easily-digestable, moisture-laden foods to try make his diet a little easier to process. Silkworms would be an ideal feeder for this.
What are you using to supplement and gutload the insects you are feeding your cham?
~Calcium with D3 M W & F, and Herptivite TR & Sat.
You should get some pure calcium WITHOUT added vitamin D3, and use that at most feedings. Vitamin D3 can build up in the cham's system leading to problems associated with overdosing. Young chameleons can be given calcium with Vit. D3 twice a week, but with a cham your age I would slow that down to about once a week. Also, the more natural, unfiltered sunshine your cham gets, the less Vit. D3 you need to provide.
The multi-vitamin supplement can also be slowed down to about once a week.
