Some men with low testosterone and/or erectile dysfunction notice that their semen volume greatly decreases, sometimes, it would seem, to almost nothing. Any decrease in function is a little scary for us men and we can't help but wonder what is going on. Look at what this poster wrote:
"One of the things I noticed that occurred along with me ED is that i stopped "shooting" ejaculate. Now it just drips out. Additionally, the volume of ejaculation is much less (about 50-75% less). Did anyone else experience the same thing along with weak erections?" 
Of course, the answer to his question is 'yes' as it is quite common for men to notice this. One of the things you have to evaluate is how much of a decrease in semen volume has occurred for reasons we will discuss below:
1. Aging. One study found that semen volume decreases by about half just from normal aging from age 35 to about age 55. The researchers found that "a peak semen volume of 3.51 +/- 1.76 ml(-1) was observed at age >or=30 to <35 years and a lowest volume of 2.21 +/- 1.23 ml(-1) was observed at age >or=55 years (P<0.05)." In other words, (P<0.05)." In other words, (P<0.05)." In other words, a man can expect a drop of about 38% at least from his early 30's to his early senior years.
2. Retrograde Ejaculation. If a man notices a more pronounced loss of volume, it may be due to a condition called retrograde ejaculation. Basically, the sperm comes up from the testes into the prostate where it is mixed with the semen. It should then, during "the Big O", get pushed through the urethra and out the opening of the penis. However, this requires that muscles on the neck of the bladder to close off tightly in order to keep the semen and sperm mixture from being shunted into the bladder.
In many men this "closing off" process does not happen, usually due to improper nerve function or even damage. (This is somewhat similar to a condition where a man loses his erection quickly called Venous Leakage.) This difficulty in sealing the bladder off is not usually a big issue for a man unless he is concerned about fertility. If you have retrograde ejaculation and you want to have kids, then you need to talk to your urologist or fertility specialist as a few treatments have been explored and some have reasonable success rates, which I discuss below.:
3. Diabetes. One of the primary symptoms of diabetes is neuropathy, or nerve damage. And one of the less well-known consequences of this type of nerve damage can be retrograde ejacaulation. The reason is that the nerves on the neck of bladder that are involved in retrograde ejaculation can be affected.
4. ;Medications. Certain medications, especially SSRI's and blood pressure medications can lead to retrograde ejaculation as well. Be sure to talk with your physician if this is an issue.
So are their any natural ways to increase semen volume? Certainly! I list these below:
1. Increase by Natural Methods. It is possible in many cases to increase your semen volume through lifestyle changes and/or supplements. For details, see my link on Natural Ways to Increase Semen Volume.
2) HCG. HCG has become a mainstay for men receiving testosterone therapy in many HRT and fertility clinics. It also reportedly increases semen volume as well in many men.
However, if you have retrograde ejaculation, these solutions are unlikely to help much. What good does it to increase semen volumen if almost all of it gets shunted down into the bladder? Those with retrograde ejaculation have to look at other options.
I do have to say, though, that semen volume is probably not terribly important unless you are interesting in having children. Retrograde ejaculation can negatively impact fertility and so, if this is your case, discuss possible solutions and aid with a urologist or fertility physician that specializes in these areas.
Some of the studies and research summaries out there show some very novel solutions: 
5) a and b combined (61% success rate with diabetic patients)
6) Improving Nerve Function. Anything that aids or cures neuropathy will like help a significant percentage of men. For ideas, see my page on Natural Neuropathy Remedies.
Anything that helps with neuropathy is likely to help as well and I discuss some of the more natural solutions in my link on Neuropathy Remedies for those interested in more information.
2) Asian J Androl, 2012 Jan, 14(1):61-8, "Male sexual dysfunction and infertility associated with neurological disorders"
3) J Sex Med, 2008 Jan, 5(1):194-8, "Medical treatment of retrograde ejaculation in diabetic patients: a hope for spontaneous pregnancy"