Might not be keeping his humidity high enough...dehydration can cause constipation, especially for BPs since they need very warm cages & don't move around much.
If you do soak a snake, you want the water about 85*; 92* is nearing harmful-uncomfortable, & often a constipated snake does better if they can swim in place (with
your support of his mid-section, & water not too deep, stay right there the whole time) in a bath tub or kiddie pool.
Post a pic of "slight prolapse" if you can. Does he eat f/t? If so, try feeding them damp/wet (to add hydration).