Building up His Stamina

Okay ladies, So me and my boyfriend have been having sex for almost a year now. We both lost our virginities to each other, and hadn't had any previous sexual partners before one another. Our sex is great besides the fact that he can only last for at max 5 minutes. He has no problem getting hard or cuming. It's just that as soon as we start intercourse he has come within a few minutes. He suggested trying Viagra so he would at least stay hard longer, but upon research we both think it's a bad idea. Any ideas as to what can make him last longer? We are up to anything.