Grandmother seeks legal guardianship of grandson with a possibility to adopt on a later date, what documents are needed?

I have a 6 year old grandson who lives outside the country w/ his father. Because of increasing issues, my son feels he can no longer deal with him. I'm proposing the idea to have my grandson move with me to Texas which is where I currently reside.
I am an older woman with limited income, what would be the best way to go about doing this....should I get legal custody or legal guardianship? Which provides me with more control? Can this be handled through my son simply signing off on paperwork? What documents would I need in order to complete the process? It's my intention to fly down to where they are & I'd like to bring my grandson back with me so I need to know what documents I need my son to sign in order for there to be no issues when bringing my grandson to the country to raise him. - Is this your question? Add additional information