As a top, one has to have a little patience. When the bottom is feeling painful, the top should stop moving and let the bottom get use to the size of the dick and let the bottom ass ease itself onto the dick. In such case, one can actually feel the dick inching slowly as the anal walls opens up to receive the dick. Alternatively, I would either gently swing his two legs (in missionary) left and right to help in the easing of his ass onto my dick. The important point here is patience and a hard dick. The bottom needs to relax and focus his attention away from the pain/discomfort. In the mean time, top can play with the nipple of the bottom or the top can be kissing or doing stuff that would stimulate the bottom while his dick is inching slowly into the bottom.