Hot weather

My son is 9 weeks old and where I live it is going to be 86 degrees today. I was wondering how I should dress him. I was going to put on a long sleeve onesie and some pants. Would that be okay? I just don't want him getting overheated or if I put on a short sleeve onesie my family being annoying saying the baby's going to get sick even though he will only be outside to go to and from the car.