Prostate enlargement is often to blame as the contractions become weaker leading to a less forceful orgasm. There are some meds that can help a little bit, so check with a urologist. Having more frequent (daily orgasms) can help also as can heating the prostate with a hot bath or shower.
Although many websites indicate increased frequency of ejaculation, a healthy diet, dietary supplements, drinking alot of water, and excellent prostate health all contribute to increased amout of semen, studies have shown no difference between the control group and treatment group. There's been no FDA-approved drug to increase the amount, either, therefore any drugs stating such are gimmicks. Unfortunately, there's no "cure" known right now. Regardless, many men experience this after their 20s and you shouldn't feel any less a stud.