Texas is one of the easiest states to homeschool in.
From
http://www.homeschoolfeast.com/homeschooling.htm:
Home schools are considered private schools and must use a written curriculum covering math, reading, spelling, grammar, and a course in good citizenship and must be conducted in a bona fide manner.
http://www.hslda.org/laws/default.asp?State=TX
EDIT: This does not, however, necessarily mean he will be accepted back into the school system at his new grade level. If you intend to homeschool your son only temporarily, it would probably be a good idea to find out what his school's position on entrance is, while you still have plenty of time to take any necessary action. Get their position in writing.