Just FYI and hopefully to cut to the chase. Some states allow family matters to bifurcate where as matters are treated / handled separately ie dissolving the marriage, custody arrangements of children, division of property, etc. If your state allows bifurcation and considering the father has created the status quo with the child in his possession (without posting the reason) there is some cause for this filing to see if the father just might, as an example, abscound out of state with the child placing the father in a very bad light with the court while mama crys "poor little me" giving the court reason to look favorably to the BM.
Assuming the father is the biological, I think it is best to proceed as normal and file for temporary custody may be using the same case number as the divorce filing. Casually mention in your filing your intent to relocate the child due to job opportunities and encourage the BM access to the child in the new jurisdiction at least until the child is older. Do not address CS in your filing at this time. The court, on it's own accord, will likely rule on CS, approval to relocate the child, etc if need be either by state statue or social policy.
Not an attorney and this posting is for your consideration. BTW, it has always been good advice to NEVER communicate with the opposing attorney .... the system is adverserial and an attorney is an officer of the court and a snake in the grass.