Most Helpful Guy
From a relationship coaching lens, a couple things:
1) Don't stay with someone because you feel sorry for them. It's leading them on and the longer you stay the longer you're delaying their ability to move forward. Today, he probably feels like y'all are moving forward, you know better.
2) Your happiness matters as much as his BUT you're focused more on his. The longer you stay, the more miserable you become, the shorter your patience and temper get, the VERY LIKEKY chance you two have a big blow up.
Most Helpful Girl
Trying not to hurt someone will only hurt you more in the end.
Don't pity him as he doesn't need it but what he does need from you is knowing that there is somebody there for him and this is the reason you feel torn about leaving him.
However, you have the right to a good life too and you need to weigh up the pros and cons of what you get out of dating him.
I noticed you didn't mention that you love him.