If he knows the material, move on. You are frustrating him and yourself. If you are afraid of missing something, have him do 3-5 problems from a section to prove mastery. If he can do them, go to the next lesson. He doesn't need to watch Mr. D if he knows the skill.
(I do like MUS for the younger grades. Try moving at a quicker pace and see if you like it better. We didn't do every workbook page for every lesson. If my son knew the material, we moved on. We covered Alpha, Beta, and Gamma in 8mos - well worth the review as it brought my son up to speed on missing skills.)
By allowing him to wallow in boredom, you risk squashing his love of learning.