I wouldn't get too concerned about venue, it may be that the county court that rendered the original order has continuing jurisdiction; (remember, residence requirements give TX jurisdiction, not the country); or, you can file in the country where mom and child reside. If mom has a problem with either one, she will have to make a motion to have it transferred to a different county, and if appropriate,the court will transfer. Not a big deal, and certainly not a deal breaker.
You do need your Court order, because you are supposed to use it in your papers to show what was Ordered, and then to explain what change you want to make. If you lost it, go to the rendering court and ask for another copy.
You can not legally withhold you son, or you can be in danger of being held in contempt. However, you can as for an Emergenct Hearing for Temporary Custody, basically making the same argument as a regular Motion for what you want, but you want it to happen asap, at least temporarily, until full notice and regular hearing can be had. Temp can happen in days, not weeks. I would travel personally down to that family court and ask about Emergent custody modification forms (for pro se individuals), also see if they have the child's affidavit form. If they don't, the affidavit is just a statement by the son, signed under penalty of law, and notarized.
You can definitely come back any time. We don't have "Board Certifications" in the legal field, unfortunately, but I do know of a Texas attorney here, although I am not sure he practice Family law, but I can find out for you.
Hope this helps to clarify.
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