Its natural. As young men we try to find a "perfect" specimen of a man in every way. So we tend to start with our fathers because we see them as great beings and so we want to see every part of them and try to be like them in every way. I wouldent think to much of it unless it gets to the point wear he is doing it in front of other guys and they start commenting on it or he wants to "try" things with you. I know when I was 14 (18 now) I had started puberty at about 11 or 12 and my dad and I have a very open loving relationship and we talked about everything. I remember at 14 I saw my dad changing in his room and had to ask him about his penis size and the such. He told me everything he thought I needed to know at that age and told me if I wanted to know more he would help me. I never needed to ask more because he told me everything i wanted to know. I would say just shrug it off or bring it up very softly. Maybe next time you are showering at the gym or some place like that and you see him looking on the drive home bring it up like " I couldent help but notice in the shower that you are moving in puberty pretty good. Do you have any questions about the male body or anything?" If he looks at you like your nasty because you looked just tell him its natural to look and compare.