"Had he contact me before forking my project I would have gladly discussed with him about improvements that could be made and I would most likely had invited him to join the project."
and
"My project is currently stagnating because I don't have much spare times and I have to focus on other things for now, that said some help would have been more than welcome."
Fork early, fork often. It's the GPL that legalizes merging, past that, the worst that happens is that someone wastes their time on code that doesn't get used. If it's the fork, tough cookies for them. If it's the "original author", they can choose whether and how to benefit from the code that has been written to extend their codebase.
If you're angry that he forked your code, fork his code back into your project. Or at least the parts of it that you want. The parts you don't, leave behind. Does it burn that you didn't get to order him around on how the project should have been improved? He doesn't need your permission, get over it.
