Honestly, I know it feels worrying but 13 months is still within a reasonable window, from what I've generally heard, most babies develop recognisable clapping and waving somewhere between 12 to 13 months, so your little one is right at that edge and not dramatically behind. The key thing people usually point out is whether the other milestones are on track, because if everything else looks fine, one delayed gesture on its own is less alarming than if a few things are lagging together. A friend of mine actually went through something similar and the paediatrician told her that some babies just need a bit more time to "click" with imitation, especially clapping, since it requires coordinating both hands at once. That said, since you already have the 15-month checkup coming up, it might be worth jotting down exactly what you're observing now, like whether he responds to his name, makes eye contact, points at things, so you have a clear picture to share with the doctor rather than scrambling to remember on the day. Either way, do bring it up with your doctor or a developmental specialist rather than waiting to see if it sorts itself out, because early input if needed is always better than waiting.