Ya its very normal, lots of guys start early. Also those stains could be involentary. But talking to him would be a good idea. his dad should do the talking though, he should let him no what is happening is natural and normal.
I am so glad to hear this comming from you and from a few other moms on this sight. My 7 year old has been having white stains on his underware for the last few weeks and my husband and I are not sure how to approach it. I do not want to embarass him, because that could screw him up for life, I am a strong believer that you really need to be a confident sexual person, and I am hoping to raise my children that way. I wish that they came with a manual!!! There is a Sexual Therapist that posted a comment to someone else that said that it is completley normal for a child that masterbates to ejaculate. So, let us relish in the fact that our sons are strong sexual beings!!!
I would get a guy he or you trust to talk to him if you think it needs to be adressed. As a guy I think that would make it easier for him and less akward.
From what I can gather, that is pre-***. My boy friend said he used to masterbate at at that age and the only thing that came out was clear pre-***. This may help. I can't figure out how it is getting in his underwear unless he is rubbing himself against something. Normally boy's masterbate with their penises out and with their hands.