Poll-What am I doing wrong here (short details)?

I'm baby-sitting my girlfriend's infant. I thought all they do is eat, sleep and poo. So it isn't sleeping, and the diaper has been full for a long time, so he doesn't have to poo. I went with food. I gave him a piece of pizza. Nope, one bite and he made like this face. So I tried radishes. I cut them thin, but no dice. Spit those out too. So what should I do, it's starting to cry again, and it's only like a month old so it's not telling me what to do for him. Thank you.