Tell your Mom. NOW. Probably nothing is going to happen. BUT, tell Mom to let her know. She may want to take him to the Dr for a check. Do the right thing. Tell Mom. NOW! Now you know why Mom says to keep small objects away from little kids. It's true.
A small marble might pass through without a problem. But .. you really need to make sure your mom knows what is going on, so SHE can decide what she wants to do ..
Just a little warning about marbles and young children .. Marbles (or anything small like that, that a child can put into their mouth) are a huge choking hazard. Don't let him play with stuff like that..next time you may not be so lucky.