The mohawk you're referring to is his hackles being raised by a process called piloerection, the same thing that happens to us when we get goosebumps. You'll probably see it many more times in his life :-)
It's a signal of adrenaline running through their body, that they are excited, interested, aroused, startled, afraid, aggressive, etc. Any number of things. The first time you see it is definitely an occasion to remember. He is so cute.