If your timing is right, and your terminology close, then what you might have seen is an Everway sourcebook. Everway was a card-based roleplaying game released by Wizards of the Coast concerning people known as spherewalkers who can journey from world to world. It could easily be mistaken for a Magic: the Gathering RPG.
To be clear, however: no official Magic RPG exists, or has existed. However, Dungeons & Dragons 5e has bridged the gap between MtG and D&D. Here's an official writeup of Zendikar as a D&D setting, straight from Wizards of the Coast. Here's one of Innistrad. But the biggest change has been the release of the Guildmaster's Guide to Ravnica, which brings one of MtG's signature worlds into D&D5E.
Ryan S. Dancey has, on Reddit, posted what can be seen as a comprehensive supplement to this answer.
Hi! I was the brand manager for Dungeons & Dragons and the VP of Tabletop RPGs at Wizards of the Coast from 1998 to 2000. I can answer this question.
There were plans to do a Magic RPG and several iterations of such a game were developed at various times. After Wizards of the Coast bought TSR, there were discussions about making a Magic campaign setting for D&D.
After the release of 3rd edition, we had planned to do a Monstrous Compendium for Magic monsters which would have been a tentative cross-over product to see what the interest level was for such a book.
In the end, the company made the decision to keep the brands totally separate. Here's the logic.
D&D and Magic have fundamentally incompatible brand strategies. This is was once expressed as "asses, monsters & friends".
- D&D is the game where you and your friends kick the asses of monsters.
- Magic is the game where you kick your friends' asses with monsters.
- (Pokemon, btw, was the game where the monsters, who were your friends, kicked each-other's asses.)
There was no good reason to believe that a D&D/Magic crossover book would sell demonstrably more than a comparable non crossover book. And such a book should be priced higher than a generic D&D book - in the way that Forgotten Realms books cost more than generic D&D books (that's the price premium for the brand). There's a fear in sales that the higher the price, the less volume you sell.
The brand team for Magic didn't want to dilute the very honed brand positioning for Magic as a competitive brand, and the brand team for D&D didn't want to try and make some kind of competitive game extension for D&D.
In the end, I think the company was well served by this decision. It eliminated a lot of distraction and inter-team squabbling at a time when neither team had the resources to fight those battles.
Today you might argue there's a different reason. The #1 hobby CCG doesn't want to be entangled with the problems within the D&D brand.
Other Wizards of the Coast employees chime in on his Facebook page.
In short:
Let him do what he wants to do, but guide him and help him with your knowledge. Make sure he understands what the chosen profession/class means and what things it offers
Long Answer:
First off, those kinds of learning experiences are actually good for him. He has a certain thing in mind that he would like to play, so why hinder him? in DSA (german abbreviation for The Dark Eye) there are many professions to choose from, and thus many different characters to create. Let him gain experience and see what he has in mind.
Now, how can you help? Have you told him what you think? Talk to him about it, ask him if he really wants to be a wilderness focused character and if he knows what that will mean in the game. Help him design the character in a way that he is a good fighter and hunter. That is (very much so) possible in DSA. It is actually one of the great strengths the system has.
You could also show him the alternatives directly. Show him what a dwarf warrior from Xorlosch (which I assume is the dragon hunter warrior you mean) gets as skills and talents and what the big game hunter offers. Also keep in mind that with a profession that does not cost as much GP/AP (depending on what version of DSA you're playing, I can't say anything about the creation prices in Version 5, as I only play 4.1) will actually allow him to still spend points on combat skills and be a good fighter.
If you're concerned that he's avoiding classes he would like more because, as you say, he's been mocked online about only playing warriors, you could also try and tell him how different a warrior in DSA is to other games, and how it is essentially an important and interesting character, not only someone who takes the front row in a fight and does nothing else.
Best Answer
You're thinking of 13th Age, by WotC alumni Jonathan Tweet and Rob Heinsoo, published in 2013. The way a character's background helps build the setting is through your One Unique Thing, and your Relationships to Icons. (Your actual Background is more about what sort of skills you're good at.)
13th Age has an SRD, and there are many sites online that reproduce it. I couldn't possibly recommend one in particular, but googling for "13th age srd" will give you a lot of choices.
And of course, the print book from Pelgrane Press is well worth the price, as it includes material and guidance not in the SRD.