How do I ask for extended time off at a new job? HELP 😩

Technically, it isn’t a new job because I previously worked there for 1.5 years. I quit because I moved out of state and that was in April, but I want to move back soon and get my old job back. My close friend mentioned to my previous boss that I’m thinking about moving back & he told her that there’s always a position open for me & to apply whenever I am ready so I’m thinking about moving back by March so that I can put in a 6 month notice for this trip. I plan on taking a trip to Thailand for a month with my brother & sister in law, she is originally from there & they want to have another little wedding down there & I would get to meet her family so this is the opportunity of a lifetime. My previous boss likes me & I left on good terms, but should I be calling him up now & talking to him about my plan even though I don’t want to move back until February or March? Will he be able to give me a definite answer for this time off before I apply? Should I take unpaid time off or a leave of absence or should I ask what he thinks? What should I exactly say to him? This trip is probably going to be from September to October. I’m having so much anxiety over this damn trip but I don’t want to wait to move back until after this trip. Any advice helps!!