Guy's perspective:
When a guy doesn't last as long as he used to, then I recommend that you foreplay alittle more. Guys usually get a pre ejactulation when foreplaying that helps them last longer during sex. As well, the 10 to 30 mins times you guys had sex, are you both going super fast or taking your time? If your going super fast, then I recommend slow down more. Remember, a guys penis before ejactulation is a pump, by having sex more and more, your building up the pump. The faster the sex, the faster the penis gets pumped up if you know what I mean.
Your problem can also mean that your doing a good job with him. For example, you know his sweet spots or positions that are making him go quicker.