Should I let him go to his dads for summer he is 18 months old?

my son's dad and I are not together so he sees our son every other weekend. (barely), but he just told me he wants him for the majority of the summer. I didn't give him an answer yet but I want to think about what is best for my son. Is is wise to let him take him for a summer? My only concern is the inconsistency of shuffling him back and forth, and his father inconsistency when the summer is over. He live 2 1/2 hrs away and even though he is suppose to see him every other weekend it has turned into every month or so. Other than that things seem fine, he lives with his parents so I am not worried about his safety.