At What Age Do You Allow Child to Live with Non Custodial Parent?

Adriannah • Married Mom of 3-expecting 4th April 2019
Backstory: my son is 8years old. My ex-husband is wanting to have my son go live with him and his current girlfriend next year for the school year. They love 4 hours away in Dallas. I have agreed to let my son go once he is old enough to make that decision however, I do not believe 8 is that age. My current husband thinks it maybe a good "eye opener" experience for the ex however, I am just not sure about this.
My ex just told us his girlfriend is 5 months pregnant. My kid have not met her nor have I. My ex, is not the most responsible of people in terms of taking care of kids. But...is an all around good guy. My concern is that she is going to be a first time mother and then on top of that dealing with an 8year old she has no relationship with?!?...I just don't know how to feel. Would you let your kid go at 8 (explain)? What is a reasonable age to let a kid go stay with the non-custodial parent?