What can I sub for the mayo?

for 6 chicken thighs, using 1/2 cups of mayo won't hurt you at 6 months out.  You need some fat in your diet.  I wouldnt eat a 1/2 a cup of mayo in one sitting but a small amount wont hurt you - you could try low fat if you want to but fat free is disgusting

I agree with some of the others, that you could do FF mayo.  I would only use the plain greek yogurt in place of sour cream because that's exactly what it tastes like (awesome on anything mexican) and it will both boost the protein count of your dish and reduce the fat.  I also am a vegetarian and was using Veganaise for a while.  It's yummy and very similar to Helman's but it doesn't contain eggs.  It would be in the organic refridgerated section at your local market.  Some people also prefer Miracle whip which is less fat than regular mayo, and Nayonaise I would say is the vegan version of that :)

Good luck!!