Came once how? Intercourse? Oral? Hand job? Other?
If from intercourse, is he doing lots and lots of stroking and thrusting in an attempt to build and peak his arousal in order to get to the brink of an orgasm? If so, then you are going about this all wrong. Peaking his arousal, peaking your arousal are both why we fool around and make out from all the Necking, Petting, Heavy Petting, that comes before ever getting to Foreplay and then Intercourse. Guys nowadays operate under the misguided misconception that the way to a climax is from lots and Lots and LOTS of stroking. Wrong.
If you mean that you have attempted to bring him to a climax from a hand job and/or oral then I suggest you read this article:
How Do I Get Him/Her to Orgasm From a Hand/Blow Job?
Guys very often are turned on much more from Foreplay activities yet do not climax. Why? Because the stimulation as pleasurable as it is is the wrong type and not what he has come to rely upon over the years to bring about a climax. What needs to happen is for him to demonstrate how he masturbates and then to tak you hand in his and in order for you to learn to mimic his movements and pressures. Key to this is for him to give feedback, verbal or non-verbal on how is is responding and for what he needs now/next. (The same is true for him with you.)
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I hope this is of help. Got questions?