I'm going to assume you're feeding a pockets pet style diet with pellets as the main protein source? He might be having harder poos because of constipation...or close to it. If he's eating dry pellets for the majority of his diet he isn't getting enough liquids. Usually sugar gliders injest the majority of their liquids from their foods.
If he's not constipated yet, he might just be having trouble moving things through because of the lack of water in his diet.
I'd recommend looking into a different, healthier diet for your babies, one that contains all the minerals and calcium needed to maintain home health. Even if you aren't seeing problems yet, feeding pellets is the glider equivalent of eating at mcdonalds every meal.
You can look up some diets like BML, OHPW, GOHPW, TPG.. These are the most common and are formulated to make feeding easy while still offering enough variety to keep your little ones healthy long term. You can leave some pellets in the cage as a snack, but they should be a snack, not a main course.
In the meantime, you can start offering more juicy fruits such as grapes, melons, berries and a wide assortment of vegetables. You can offer some plain canned pumpkin to soften things up, of avocado. Generally it is 1 tbsp fruits 1 tbsp veg per glider, per night. Protein sources follow the same structure: 1 tbsp per glider per night. Boiled chicken, cooked ground turkey, lean ground beef, boiled egg, full fat yogurt or cottage cheese are some great protein sources your gliders will love to try.
You can also offer mealworms, hornworms or superworms for bonding treats in the daytime as well.