My husband is having trouble during sex and now is insecure

My husband and I got married a few months ago and before then we had been having sex regularly since November 2015 and he wanted to have sex all the time, he was fit and never had a problem performing. We decided as a couple we would go a couple months without sex until we got married so we stopped in July I think and once we got married in September he didn't have a problem either but we both stopped working out regularly because we had moved and wedding planning was time consuming. Well seemed like a month after we got married he started losing his sex drive. He didn't push for it as often and we had sex about once a week. And last week he started working out again and that night he couldn't preform. Couldn't get a boner. He is now super insecure and no matter what I do he can't get a boner and I know he's attracted to me. He's only 23. We aren't sure what to do. How can he change this?