Thief abilities were always stupid. We realized that when they first came out. The simplest answer is just to add 50% to everything, then subtract 10% for each level the target is above the thief or add 10% for each level below. (Use dungeon levels for traps, etc.)

his Move Silently and Hide in Shadows skills are quite low to the point that he rarely ever makes the roll

That’s a big problem with almost OSR rules. Thief abilities need to be opposed checks by level of the opponent (or object).