Your little egg is doing just fine, and he will do best if he can continue to work on it himself. That's great that there's an "external pip" (that's the word for the little crack/hole he's made). He is now going to take a break and reast, breathe, and absorb the remainder of his yolk before he goes back to work. His "break" may last as long as 24 hours. So don't get antsy!
Meanwhile, keep the temp as close to 98-99 degrees as you can, and the humidity high (I'm not sure the right humidity for hatching chicks--I have ducks and they have different moisture requirements). Don't handle the chick more than you have to. Listen from time to time--if he is cheeping and it stays the same or gets stronger, he's fine. If he begins to weaken, you'll notice his cheeps weakening. Don't jump in and help, but pay attention--if he continues to weaken, then it's time to help.
If all goes well, and it sounds like it is, then the next thing that will happen is that he will widen the hole he's started. Then he will begin to "zip" a circle around the top of the egg, to make a little "cap" that he can push off the top. Then he'll stretch and stretch until he pops out.
But that may not happen today. It might be tomorrow some time. So be patient!
The good news, though, is that since he has pipped externally, you should be able to help him successful if the time comes and you need to. Any time before the external pip can be very dangerous. It's still dangerous, but usually has a high success rate. Good luck.
