If your developer wants to learn to develop add-ons for XF2 there is a documentation section where you can learn the basics of the process:
https://xenforo.com/xf2-docs/dev/
There is also a support forum with a lot of technical questions answered so you or your developer won't be on your own in the learning process. There's also a custom development forum where you can hire experienced developers for a fee to do all kinds of jobs from system administration to plugins and custom themes.