1) Can such important detail about family be a typo mistake?
To be honest, I don’t think so. Years ago, I’ve experienced dating websites and from my experience, if you really want to find that someone special, you want to be sure that the info you’d put down in words are right.
I don’t buy the idea that his account could be hacked. Why would some change just couple of information about his profile?
2) What would he reply? He will for sure say, it was a mistake?
He could reply saying that it was a mistake. What I reckon is, he will reply something saying that it wasn’t him or if it was him, he’ll manage to find a way to show it was a mistake.
3) How to know if he is lying?
I don’t think there’s a 100% effective way to discovery if he’s lying.
I think you should ask yourself something like, “Do I really want to start any kind of relationship with someone that I can’t trust even 10%?””
I think it’s ok to keep talking to him, but just be aware that he could start saying things that you want to listen, but aren’t the truth.