My dad wants me to be his best man at his wedding but I feel like I'm not in the position to be best man and that he is just taking everything to fast

So last Sunday my dad proposed to his girlfriend of around one and a half years and they are currently in the planning stage of their wedding. They already have the venue picked out which I though was a little early but it's been a venue his fiance had talked about for years so it was going to be a given. However, my dad, a week into planning has already started asking friends to be his groomsmen and he asked me to be his best man. The wedding isn't until late April and I feel like he is taking everything way to fast. I'm only 15 and by the time of the wedding I'll be 16 and almost a month. I just feel like I'm not in the position to be his best man. He's had friends who have been there for him for years and he's known since high school but he asked me. I told him in a nice way that he was kind of taking everything too fast but he just shut down and got sad. I don't know if he is actually sad that I didn't want to do it or if he is over-reacting because my great grandmother is about to die and my grandmother isn't in the best of health either right now so he is just taking things a little too gard. I told him that I would be happy to be a groomsmen or at least participate in a substantial way with the wedding because I am genuinely happy for him and his fiance is one of the nicest people I've ever met but I just don't think I'm the best choice for best man. I know nothing about weddings though so I have no clue how to handle this situation. Sorry if this is long winded