Need help/advice!!

Okay so my boyfriend and I have been best friends for about 5 years and we started dating about half a year ago.. so today we were at my house and we were alone. We have never had sex together nor have we been even close it but today We started making out and..well you know got naked haha but when he was about to put it in, his boner went away completely... keep in mind he's a virgin but I am not so my questions is why did his boner go away?? Did he get shy? .. or was he not into it?? Like please help !!