I found this on jackinworld, maybe it will help.
Originally Posted by jackinworld.com
This is actually a common experience in boys still going through the early stages of puberty. There is an internal mechanism that prevents urine from being expelled during ejaculation â€” but many guys are able to ejaculate before this mechanism fully matures. When this happens, a small amount of urine (rather than semen) may be expelled with orgasm. The condition will go away as you get older. If it's still happening by the time puberty is near completion, consult a physician