It's really not a problem... it's just that physiologically he needs "recovery" time.
The "recovery" time is highly individual, so some guys can continue just a few minutes/hours after ejaculation while others need more time. So it is probably a natural "pattern" of your boyfriend. Maybe he could try to satisfy you by other forms of sexual stimulation which do not require penetration - e.g. oral sex etc.
I've had this problem with my fiance twice since we've been living together, and we've worked out that it is tiredness but at a subconcious level. It's his body's way of saying 'Pack it in, I need rest', give it a day or two without sex and you'll find 'once a night' will probably turn into twice or maybe three times. Good luck ^^
Since talking to him, we've tried the method of him stopping during sex when he's about to *** and then carry on.
It's worked! Hehe we have longer and MORE sex in the past two days. :D
Thank you guys for your advice :) x