My man can't last

When me and my boyfriend first got together 3&1/2 years ago he could last forever and within the past probably 1&1/2 he can't last over like 5 minutes. It doesn't matter if there's foreplay or not. Should he be seen by a doctor or is there something we can do to fix this? The rest of our relationship is perfect. He is 21 and I'm 20.