You can always try a "sauna" too, to help with shedding. Put him in a plastic container (with holes for air) with warm wet paper towels in the bottom for a couple hours.

Fasting snakes are a serious pain... I'm going through this with my male hognose right now.. I think he might actually starve himself to death.

That said, I have a couple questions / thoughts for your snake.

1. Is he eating regularly now?
2. How often are you feeding him?

Although my advice may change depending on what you say for my questions I have a couple thoughts that might help...
1. I have 2 female KSBs and when I noticed they weren't gaining weight (while eating regularly), I began feeding 2 pinkies at a time. This helped them gain weight until I upgraded them to the next food size.

2. You can try feeding them every 5 days (again, not sure if this is what you're doing). Mine have no problems eating every 5 days and are growing like weeds (I got a female in Oct. that weighed 15 grams, now she weighs 35). You may want to try this before trying feeding multiple items per meal.

3. What is the enclosure temp? You may want to increase to 95 on the "hot end" if you don't have it at that temp already.

4. This is a long shot... Do the snake's poos look "normal" or odd (runny, etc.)? It is possible that it could have some sort of internal parasite like worms that would keep it from getting all the nutrients from food.

Hope this helps!
If you need more advice, I know of a sandboa only group on FB that you can try: sandboa keepers.