I would say something, nicely though. I would say he doesn't know how to handle it. Also just wondering how old your dad is? Because with older generations often miscarriages and still births, and passing of babies and children was something that was really taboo and basically not something people would talk about or acknowldege.

I believe this is far from right but back not too many years that was how it was so maybe your dad just doesn't realise that Taite and Seth's birthday is something that can be acknowldeged? Maybe you could just tell him you want to remember and celebrate their lives and tell him how you want him to be a part of this.