Do I need a stroller to travel with 18 month old baby?

My husband, my mom, me and my (going to be at that time) 18 month old baby are traveling to mexico for a family vacation this summer. Multi-city trip with layovers. I have a big travel system stroller, but it is weighs almost 30 lbs and waaaay to bulky to travel by plane with it. I have my Ergo baby carrier which I use often for shorter trips. I was thinking to just take the Ergo and then have him walk or have my mom or husband hold him when my back needs a break. BUT do you think the summer mexico heat will be too unbearable w/ the Ergo? Also I will be about 22-24 weeks pregnant at the time, so depending on my size I may only do back carry. Do you think I should spend money on a new stroller just for this vacation? We will also be lugging around a big carseat so we are trying to downsize on stuff to take. If you suggest taking a stroller, any light stroller suggestions?