Have you tried adding BRATY foods to his diet?
B - Bananas
R - Rice
A - Applesauce
T - Toast (dry, white)
Y - Yogurt
These foods are all constipating if used in excess and I have been told by several peditritians that this is the best way to slow down diarrhea.
Also, when I was a kid, I had a really long list of allergies - among them sugar, cow milk, goat milk, soy, rice milk, nuts, chocolate and tap water. I was treated for many of my allergies with NAET (Nambudripad's Allergy Elimination Treatment) accupressure/accupuncture treatments. Thanks to those treatments, I am virtually food-allergy-free.
Splenda is "made from sugar, so it tastes like sugar" but that doesn't mean it is natural - or even good for you. The sugar is processed to remove part of the chemical makeup that causes our bodies to process it as a carbohydrate and provide us with energy (calories). My own personal opinion is that Splenda has it's place in some products and for some people. For someone who is diabetic, an occasional treat made with Splenda could be enough to keep them sane. I consider it to be a "food additive" rather than an ingredient I would want to ingest as a staple of my diet on a regular basis.
Stevia is a better non-caloric sweetener choice, but it can be rather expensive.
Sugar has very little, if any, nutritional value as it is and should be used sparingly. Where is he getting most of his sugar? Fruit juice, fruit snacks, cakes, cookies and ice cream can cause diarrhea for other reasons than it's sugar content.