Marry the partner, adopt the child, divorce the partner.
There's a BIG difference in becoming a legal guardian and just having a Thai change their birth name.
First names are a snap and are done regularly in Thailand but a "Family" name opens a whole LOT of legal issues. Birthrights, Military Lottery, Thai Citizenship Rights, etc.
My father did it with his ex and my newest, youngest 'brother' but he was married, obtained visas to the states for them both, got her citizenship and THEN adopted him with a name change (First AND Last).
Question is, to do it now, what are YOU willing to do?