At 18 weeks he is still full of puppy fluff and I suspect that this is what the brown hair you can see is
It's probably not ready to give yet but you can try
Use a pair of Marigolds, I cut the fingers off mine and just use the finger parts. Get a little bit of hair between your finger and thumb and pull, I would imagine that you probably won't get much out yet, but you can get him used to the sensation by doing this at an early age. It's very satisfying learning to groom yourself
be patient this woolly mammoth stage deosn't last long and you will find when it is ready to give it will come out fairly easily