Are you sure it's a scab and not something stuck on to his skin? Can you post some photos? You can upload photos here https://www.beardeddragon.org/useruploads/ and then add them to your thread using the XIMG button.
OK that's definitely a scab or a growth of some sort. How long has it been there? Has it appeared to grow or spread? It may be healed over from an injury or it could be a growth. If it looks like it causes discomfort or if it looks like it's growing or spreading I'd bring him to an experienced reptile vet for an exam.
Just noticed it a couple days ago. Doesn't bother him to touch or anything. He is quite a jumper and explorer. Has little black/gray spots all along the underneath of his legs