It should be fine. I add probiotics to my Ds1's expressed BM that I have in the fridge the day before giving it to him. It doesn't seem to effect anything. Probiotics are naturally found in yogurt as well... and that's mixed a long time before you actually consume it.
The reaction you're looking for is also called "die off", when the probiotic has reacted a therapeutic level, the pathogens in the gut (aka Bad bacterias, fungus, parasites ect) start to die and they release toxins into your body. They will often come out in the form of rash, hives ect. He may also seem to have increased food sensitivities. Also watch for irradic mood changes, defiance, sleep disturbances ect... all can be signs of die off and that the therapeutic level of probiotic treatment has been reached. That's when you know to back off a little and not to increase. If that makes sense.
To help with the detoxing process, daily detox baths are key. Put 1 cup of either Pure Apple Cider Vinegar, Baking Soda or Epsom Salts in a warm bath. Have him soak for minimum 20 minutes. The longer the better. The detox baths will help to restore the skin's PH and open the pours to allow the toxins to come out.
HtH