How do we keep da stamina?

Well iv been talking to this guy bout a month before mothers day and dating since mothers day. He really likes me alot but when we tried to have sex his penis was hard during foreplay but fell when was time to perform. This has happen 2times and the third time I tried using a third person (female) this tome it never even go up. He has a big one and I would like to enjoy him, what do u suggest he do or what can u do to keep his penis up once he get it there?