First of all, don't be so hard on yourself! The next few times you dress and undress, pay attention to the details of how you do it rather than just going through the motions. Then you can help walk him through the steps. If he has two-piece sleep clothes, you can lay the pants on the floor (with the tag at the back/bottom) and tell him to put one leg in each side of the hole while he is sitting down. Once he pulls them up as far as he can, have him stand up to finish the job. For the shirt, you can lay it on the ground with the tag facing forward/up. Then have him pick it up and pull it over his head, getting his head in the big hole first. Then have him reach his arms up and a little to the side to get one arm in each of the arm holes. Another option is to have him watch you and/or your husband get dressed and undressed (for real or with larger clothes over your regular clothes if you'd prefer) and imitate you. Good luck! You will figure it out and so will he and you will be laughing about it soon. 
