When Can Baby Wear Shorts? (Long)

Betthany • Stay at home wife and mother. Married my best friend Nick 6/28/14 and gave birth to our son Noah 1/4/15.
So, this may seem like an odd question, but I'm a first time mom. I was just wondering, at what temperature/kind of weather do you dress your baby in shorts? My son is about 3.5 months old, and we are going to a cookout on Saturday. The weather is supposed to be: mostly sunny, 75 degrees Fahrenheit, and hardly any wind. I personally would wear shorts, but I don't know if it's ok to dress him in them? He will be covered with sunscreen, hat and sunglasses to protect him from the sun but do you think it's warm enough for him to wear shorts? I plan on taking a small jacket or hoodie and a pair of pants in case it gets a little chilly for him.