I think it is a huge step that he admitted he has a problem. I personally don't think there is a problem with going to the first meeting with him if it is an open meeting. The treatment center my husband was in encourages family members to attend an occasional meeting with the addict. Hopefully he will be able to go to the second meeting alone.
I understand being confused between supporting and enabling and being codependent, but in this situation I think the first meeting is ok to go with.
Good Luck and best wishes to you through your very difficult time!!!