When i ejaculate there is hardly any semen, is there a way to boost sperm volume with a vitamin supplement?

Not really: A healthy male will produce just what his body is to produce. Unless you have some hormonal or genetic problem or other issue, there isn't anything specific to prescribe. The testis, prostate, seminal vessicles and other glands produce a cerain amount at a certain rate. If you have poor nutrition, a multivitamin/mineral may help. Stay hydrated, stay healthy and cut back frequency of sex.

While you do not: Reveal your age or hx of treatment, younger men produce more copious semen than older men. In my prostate cancer population (men over 50) treated with radiotherapy seminal fluid volume decreases, and to my surprise, become more aqueous. Most potent men remain potent in follow up. Patients treated with prostatectomy have no semen.