Yeah he's a bit of a wimp, but he made that one dudes head in Africa explode by simply spitting on his face. He knows something is wrong with his body and he doesn't know what would happen if he blows his load. Penis spirit also kinda sucks in explaining things, but I guess he's just too primitive.
Mc just doesn't want to kill. It's one thing to kill someone in some war torn place in some country in africa but another to kill a girl inside love hotel in big city in japan.
Also he was literally loser incarnation so far until he got changed into omega species. Him becoming some giga chad in few chapters would feel a little off and cheap, instead author is trying to show process of him changing into that or so I hope.