Well, there are a couple of things which could be happening. Some guys just don't produce much semen. You could be one of them. But, it could also be Retrograde Ejaculation. You have all the sensations of cumming but the semen is firing backward into the bladder, instead of out the end of your cock. You might be able to check this one at home. You'll need a clear jar or bottle. Drink lots of water until your urine is clear. Then you'll masturbate to orgasm.
When you take that post-cum piss, use the bottle.
If this is simple retrograde ejaculation, the urine will be cloudy...that's your semen. It's not dangerous in any way but, at this time, there's nothing anyone can do to change it. The sphincter that closes off the pathway from the bladder is either not there, or not operating efficiently.
Two: You don't mention it, but if there is a small bulge, or swelling, in the lower abdomen - beside the base of your cock - right after you orgasm, there could be a blockage of the seminal ducts. The swelling can ache but will usually go away after a while as the body absorbs the semen. Again, you get all the sensations of an orgasm, but no true ejaculation. It's a long-shot but vibration therapy might be able to clear it. Or, micro-surgery could remove the blockage to fix this, but you need a very skilled surgeon.
If you want to sire offspring, your sperm would have to harvested. There are several methods, but your situation would most likely involve a procedure known as Microsurgical Epididymal Sperm Aspiration (MESA). There are other methods which you can read about here: How can sperm be retrieved.
In a strict case of retrograde ejaculation, there is a centrifuge system of collecting sperm from urine. It involves jacking off
and then pissing into medical-type urinal large enough to capture everything.
Though painless it's difficult to separate the viable sperm from other cells and not very successful. If you determine you need to see a doctor, I suggest you create an opportunity to broach the subject with your father before seeing a doctor.
That way, he can inform your mother in a way that won't have her questioning you.