E.H.
At such a young age, the guidelines do not favor overnight stays for the non-custodial parent, if that makes you feel any better.
The letter of the law is:
§ 153.254. Child Less Than Three Years of Age.
(a) The court shall render an order appropriate under the circumstances for possession of a child less than three years of age.
(b) The court shall render a prospective order to take effect on the child's third birthday, which presumptively will be the standard possession order.
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Usually the court interprets the phrase "appropriate under the circumstances" to mean that very young children are not expected to spend the night away from their primary caregiver. This will at least give you until he is 3 years old before going to a more equally divided schedule.
Also, hopefully it will discourage the father from moving back during the summer just for partial custody, since he won't be able to keep baby for a long period of time continuously, but rather he would likely have to return your son to you every night.
Good luck, I know it's a tough situation to be in.