My husband left me and is keeping my child from me, can I go get him?

My husband left me in January and has since kept my son, who was only 2 months old at the time, away from me. We are stilled married and haven't started any divorce or separation proceedings. He's basically just taking control and telling me I can't come get him or see him anywhere other than his house. I don't think I need his supervision. My son's birthday is soon and I'm thinking about just going to my husband's house and demanding my son. We have so many bills and I don't have the money to file for divorce now and he knows he's better off with things the way they are now so going that route isn't an option. I've missed my baby's first Valentine's Day, Easter, Summer and pretty soon his first Halloween. Please somebody tell me my rights. Can I go get him? If he refuses to give him to me, can I call the police? Could I go get him at my mother-in-law's house while my husband isn't home (she wouldn't cooperate unless the police made her)? Please just tell me what I can do.