no problem cumming through sex ...

Ask him to tell you. Just like he cannot guess what pleases you, you cannot guess what pleases him.

Two techniques that can help is using your hand around the base of his penis, giving a hand job while your mouth or tongue is on his head. The other technique is not for everybody, but it is very pleasurable for a man - deep throating. Push his penis inside your mouth to the point where it goes into your throat. This will cause a gag reflex from you. If you can control that, it will be very very pleasurable for him.

Do not be afraid to use your tongue and hand to stroke and stimulate around his penis, scrotum and anus. These are all sensitive areas. Ask him to tell you if he does not like something.

But ultimately he can tell you how excited he gets and what you can do to increase his excitement to the point where he ejaculates.

Mostly the mouth provides heat and some pressure - although oral is a very sexy thing to do the lips, because they are so smooth, provide so little friction it is unlikely to result in ejaculation. This is why it is important for the hand to be busy at the same time...

If you can use your tongue cleverly, energetically and for long enough, especially around and just below the glans he should be able to achieve orgasm without any bother if he is sufficiently turned on - nothing is more of a turn-on than the woman being enthusiastic about what she is doing - which is why the 69 position works so well for lots of people especially when you are used to each other...

Alternatively, if you position yourself so he can reach your clitoris with his hand while you are performing oral on him, he can stimulate you - you should find if he keeps you just below your orgasm your tongue, lips and hand will develop a mind of their own...
Or you can stimulate yourself this way...

Gently squeezing and massaging the scrotum is very pleasurable. Try applying a firm pressure around the base of the penis - this will often increase the sensitivity of the penis as will applying a firm upward pressure just behind the scrotum...
Pointing the penis down towards his feet while you perform sometimes increases sensitivity also...

All I can say is your BF is lucky that you do this for him.