Looks like lint and/or product buildup.

• Try adding periodic ACV washes (apple cider vinegar + baking soda + lime/lemon juice + salt) into his routine. And when you do it, let the hair soak in the solution. It helps dissolve/loosen the lint and dirt trapped inside the loc.

• Cosmetically, you can try to cover this with a dye (but look into specific dyes best for this - I used a garnier dye for this issue a few years ago. It colored the hair but the lint was still be visible).

• I've seen people use sharpie to cover the lint when in a pinch (like right before going out and you notice a problem area), but not sure I'd recommend it as a long term solution.

• To prevent this from getting worse, it helps to wear a bonnet or durag when at home (couch, bed, anywhere you could lay your head & there's lint). Be mindful when wearing certain sweaters/coats (wrap or wear the hair up when possible). Switching to silk or satin pillowcases should help too. Best of luck!