Step parenting

My (step)son is coming to stay with me for a few weeks. His father is in military and away for now so it will just be us for a bit. We were best buds! When he was younger he used to call me mama. More so because I was who took him to daycare and fed him while his dad was at work. Now he's gotten older (5)  and it's a been a year since I have saw him. I'm not sure what he should call me. Me and his mom are very friendly, usually talk for the arranging visits and have a good relationship. She has remarried and he calls her husband dad as well. I am not sure what he should call me. I don't need him to call me mom I love him all the same but I don't like children calling adults by their first name. It's a respect thing. Also as we begin to try for our first child, I don't want my children used to hearing me called by my first name.  I will let him lead and call what he is comfortable with but should me and him talk about a special nickname for me instead of my first name and mom. I would be okay with Ms.Jacy but I feel it's too formal for our loving blended relationship. Please give your input.