Birthsign is personal preference.
For example, my Breton Healer uses the Apprentice Birthsign for a number of reasons (Yes, I know that it's negates the Breton's strongest (IMO) point, magicka resistance, but that doesn't matter). For one, she'll need the magicka, and I didn't want to use the Atronach. Two, the Apprentice's month is Sun's Height which corresponds to July, the birth month of the IRL person I based her off. Three, you can coutner any weaknesses mid to end-game if you so wish.
That said, stat boosting birthsigns are a bit of a waste IMO, as by the time you get to the end-game it'll make no difference (unless it's the Theif which boosts luck by 10, handy if you'd like to get luck to 100). I suggest choosing one where the special effect suits your play style/interests you.
For roleplaying, I suggest putting three easily controllable skills that you use in his Majors, and filling the rest of the spots with skills you won't use. That way you won't increase past level 25/30 (at level 30 you can obtain everything in the game IIRC), and you can use your skills as much as you want.
Alchemy might be a bit of a no-no in majors as it levels so fast. But if you use it normally/sparingly, you'll be fine.
Maybe, make him a sneaky character? Light armour, marksman, sneak (NOT IN MAJORS, levels too fast for my liking) and alchemy.. Then the rest is up to you.
As for the roleplay, let Tobias talk to you. Give him the bare bones of a personality, and in time he'll tell you more and more about himself. It's a hard concept to get your head around, but try it, and if you get stuck, we'll always be happy to help.
Tl:dr? I apologise.
Also, the above is all my own opinions. Feel free to take as much or as little as you want from that