How can I bond with my step-son?

Thats a complicated situation and Im sure it puts you in a really awkward position. I would just keep doing what your doing, and doing little things, spending time with him, talking about his day and letting him come to you if he has any problems or wants to talk. You could even ask how is Mom is doing..... She shouldnt make him feel bad about caring for you because its not about her and its not fair to him. Just make sure you and his Dad dont have any negitive conversations about his mom or your feelings on the situation when hes anywhere around (which Im sure you dont) and try to show him your a even party in this. If hes having a problem with his mom try to give him advice on how to fix the situation and not show any feelings you might have on the situation and he will not only respect you but hopefully in time she will see. Im really sorry she guilts him though. Thats totally unfair of her.

Depends on the age. However, I would try to find one way that you can really contribute to his life, and capitalize on that. For example, are you really good with homework? Are you a really good listener? Do you like monster trucks, or spy movies? Do you make great desserts? Whatever it is, make that your special thing, and build on that. That way. you don't look like  you're competing, you just look like you have a special bond.