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.
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.
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!!!
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.