That is a sperm plug. You can very gently pull it out. It is pretty gross but most of us have had the pleasure. Ugh. Let me know how it goes for you. If the thing just won't come out try soaking it for a while, that may help. How long has he had it? Do a search and you will gets lots of info.
2.5 Quads, 2.2 werneri 1.1 , hoehnelii