Are his poops strung together?
If so add a bit of organic pineapple with no real or artificial sugars added plain to his diet. This was recommended to me years ago by a licenced squirrel rehabber, as it aides in digestion.
Also Pet Flora from Safer Medical of MT. This is actually a very good deal, as this source will last for nearly 4 years of daily doses of 1/64 Tsp. per day added to the diet (in plain organic LF yogurt).
https://www.amazon.com/Pet-Flora-Spe...A2XUOVLOF1KYDU
https://www.amazon.com/gp/product/B0...=ATVPDKIKX0DER
I recommend adding Milk Thistle seed extract to his diet; it lends support against toxins.
Just 0.05 ml measure perhaps in organic yogurt
(Stonyfield Low Fat Plain creamy
http://www.stonyfield.com/products/y...fat-plain-32oz
(Green Valley Lowfat plain)
https://greenvalleylactosefree.com/p...e-plain-yogurt.
Also, providing a good water source to his food to support better hydration.
For elimination of other polutants and heavy metals, I recommend Aquasana affordable bottle filter.
https://www.amazon.com/gp/slredirect...getName=sp_atfRemoves over 99% of bacteria, lead, chlorine,
cryptosporidium, and giardia from any tap water.
https://www.amazon.com/Aquasana-Clea...r+bottle&psc=1