but no matter how much coffee I gave him at the second spot he just kept asking for more and wouldn't move on again
Ok, correction, after an enormous amount of coffee he continued. :)
I got to the skull castle at the end, and triggered the boss battle.
I was doing pretty well (because I had a lot of power ups), but got hit by a rat nevertheless and got respawned at the respawn location with the boss rat gone. Moving towards the end of the level again, I found the guy on his way back asking for even more coffee, so I bought him one more for the road, he moved forward to the end again, and I got the text saying I won the game. :D
Not sure what happened to the giant rat boss. :)