You should definitely reach out to your Foster Coordinator or Foster Mentor, etc, and see what the organization's protocol is for constipated kittens (especially if he's still constipated now; way too many days). They may want you to bring them in for medical attention.
For the future, I've also found a little pumpkin puree can help - Tiki Cat sells one but you can use any *totally unflavored* pumpkin puree. Also massage/stimulating while in a warm butt-bath (versus just stimulating like normal).
Hope kitten doing better!
#PetHealthandNutrition