Can you wiggle the head of a flathead screwdriver in there, if you can wiggle it around and maybe twist it a bit to try to loosen the knot. Be careful to get the screwdriver head between the ropes and not in the rope.
I've had luck doing this before with knots in my rope that are super tight
Give the saddle to your son too so you have an excuse to buy another one.
Or cut it off and I’ll give him a 9mm HTP