I would just tell him (by email? Not sure how you communicate with him) that you think you're ready and able to take over management of your investments on your own now, and thank him for his time and management up to this point and ask how to take over your accounts and stop the professional management. If you're leaving your account with the firm but taking it out of management, really, that's all you need to do. If you're speaking to him, just keep it light and express your excitement about learning about investment strategies and that you're really wanting to get "hands-on" now - it's not about you're firing him, it's about you gaining the confidence to do it yourself now (even if it really isn't).
Of course, if they guy starts giving you grief about it, that is a whole other story... I had one adviser that I ended up reporting to the parent company because he started hard-sell tactics pretty much as soon as I got my account (inherited several professional managed accounts with different investment companies - so pressuring me about giving him more money right after my dad died), and when I told him finally after a few months that I was planning self management, got told that this was equivalent to a doctor hearing about a patient wanting to operate on themselves... i.e. I was too unskilled and ignorant to do as what he did. He skirted the line to calling me stupid, but that was pretty shitty in my opinion, and I definitely didn't get that treatment from my other adviser - who was very encouraging and agreed index investing was a very solid path to good returns while minimizing costs and was very nice right from the start.
I did the switch to self manage by phone with my (good) adviser, but I don't have any worry about them referring or not referring clients. In any case, it was pretty simple - I called him up, told him that I'd finally gotten up to speed on investing and what I wanted to do, and was ready to take over my investments and get everything out of professional management. He walked me through setting up the self-managed accounts (they had different account numbers/setups depending on if it was pro or self managed, so we had to shift all my stuff into new accounts). Took about 2 days total start to finish for the funds to move over, but I didn't have to do anything other than the phone call and open the new accounts online (took about 15 minutes total for all of that).