A.J.
My son has SPD as well and we decided to wait to worry about the training and just let him use the pull ups... by the time he was 4 he was potty trained and by 4.5 at night as well... I would not stress about it too much he will eventually get it he just may not be ready. LOTS of boys with out SPD are not trained until the age of 4.
HTH
A. J