Try to make seeing other people on a walk a poitive experience. Right now, you know he may snap, so you probally tense up and he senses that. Some dogs tend to feel more protective/aggressive when on a leash, and sometimes that comes from signals when we are giving them without realizing it.

I think this is what I would try:

When you see another person approaching, put Tobey in a sit, get him to focus on you, using treats, praise or whatever gets his attention. Keep your voice happy, talking to him and praising him for being a good boy. Once you have done this (days, weeks whatever it takes), don't stop him, but still keep his attention on you as you walk past the other person. Praise him, talk to him, keep a treat in his face if you need to (cheese, hot dog, something high value). Gradually you can wean the treats away, just keep talking and praising. Get his focus on you.

Good luck!