Warn your husband that if he doesn't stop that you will call social services on him. Warn him ONCE and then follow through with it. Tell them that he won't stop giving your kid beer. Social services will probably show up to your house talk with your husband and tell him if he does it again he will be arrested. Then if he does it again, have him arrested. (Don't worry, he likely won't get jail time). Doing all this proves to him that you are serious about your son not drinking alcohol at this young of an age. The implications could be very dangerous, not to mention it's extremely illegal. Hopefully you won't have to take it this far and he will stop with the social services.
Bottom line you need to take action. Not another drop for the kid NO MATTER WHAT. Warn your husband of what you are going to do and then DO IT!