I don't see why it should be any problem. I am a single mom of a two year old and we have flown together many times in the last year and a half. Your husband and your mother-in-law together should have no problem handling him. My advice to keep him happy on the plane is to bring lots of books and lots of snacks. Always keeps my son happy.
As far as separation anxiety goes, he'll miss you but he'll be fine. When my son was about a year old, I left him with my mom and my sister for about 3 days while I flew down here for a job interview. We missed each other but talking on the phone helped. He was a bit mad at me when I got back but he got over it very quickly (I'm talking within 30 minutes). Kids are tougher than we think and as long as he's with someone he is comfortable with he'll be fine. My advice is to not worry about it too much. Kids pick up on what you are feeling, so he's much more likely to cause a fuss if you are acting like it's the end of the world. If you're calm and excited about it, he'll probably handle it much better.
I agree with the advice to make some plans for yourself. Not only will you miss your little guy but you may as well take advantage of the opportunity to be a free woman. So, call up your girlfriends and do something fun...and make sure you get some extra sleep. If you're like me you could use it ;)
As far as your mom goes, just tell her that you appreciate her concerns but you and your husband feel like this will be ok for your son. You will certainly be willing to listen to her advice and take it into consideration but in the end you are the parents and will make the decisions for your child. If she brings it up again at that point just tell her you've made up your mind and you don't want to talk about it anymore. Then change the subject. She might not like it at first, but she'll get over it.
Good luck,
B.