apples are not a laxative.
The BRAT diet stops diahhrea. BRAT = Bananas, Rice, Apples (or applesauce) and Toast. Don't give him any of those things.
Personally, we have had great success with either STRAIGHT apple juice (the juice is not constipating like the fruit is), OR straight prune juice. Do not dilute it with water. And give like 2-4 oz. You'll get a nice huge blowout, usually within 6 hours or less (IME). And a happy baby boy.
IMO, this is way less invasive than a suppository or mirilax..... although still not my fave thing to do b/c of the sugar in juice..... but if I have to pick something, this is my go-to method for constipation.
If he is nursing, you can also drink some Smooth Move tea yourself and then nurse him, he should get some from you (accdg to my MW when my YDD was constipated). Or perhaps (if you ask some medical professional) you could offer him a small bit of it, too.
Hope he feels better soon!