What would you consider sufficient notice to book airfare for Christmas visitation out of state?

Keep looking for a cheap flight, and tell him you're willing to split the entire cost in half, not pay for the more expensive part. You have the kid with you, it's not like he can make you comply. In TX, the parent who moves away bears 100% of the travel costs. A round-trip ticket is cheaper than two one-ways, and two months is ample notice. You could ask the airline or a travel agency.