You can't build for this on your own, or with our help

It sounds very much like the DM is using 3.5e rules as a toolbox to create a home-brewed game that works somewhat like the older AD&D 2nd edition did (sneak attacks, XP, and rogues in general work in 2e as you describe these house rule working).

I can understand the motivation—more streamlined rules, keeping a desired playstyle—but I have to question the DM's wisdom in just dropping you into this game unprepared. It's not very welcoming to surprise you with a change of system.

What do to do: recruit your DM's help

You want to stay in this game. I 100% guarantee that nothing anyone here says will get your DM to adjust the rules to work how D&D 3.5e works as-written, because it's clear to me that he's made these changes to 3.5e very much on purpose. (Or rather more likely, made a few 3.5e-inspired changes to 2nd edition AD&D on purpose.) He's not going to turn this ship around now.

So your only options are to leave, to stumble along trying to figure it out, or to work with your DM to figure out how to make a character you will enjoy in his game.

I don't recommend the second option, since that's unlikely to be fun or work well any time soon unless you're familiar with 2e already. You don't want the first option, because you really enjoy the game itself, otherwise. (Keep it in mind as an option though—not every game is for every person, and you need to keep an escape hatch if it all goes wrong. If you're prepared to leave if you have to, it's easier to leave gracefully.) That leaves the third option.

Communicate your problem and willingness to move forward

Let him know that you enjoy his game, but were working with a mistaken impression about how characters worked and you're not interested in this view of thieves. He probably (knowing 2e DMs) has his own procedure for changing characters, and getting that process going now will get you into the game sooner and happier than trying to sort it out on your own.