Pity it can't be Regulus since he died about a year before Dean should have been born
Actually it could be.
Regulus died 1979. The age range for Harry's year was from Sept 1 1979 to August 31 1980.
There are 40 students in Harry's year. 11 months divided by 40 students is 3-4 per month.
We don't know Dean's actual birthdate or Regulus' death date (or birth date).
Given a nine-month pregnancy, Dean could conceivably (heh) be Regulus' kid.
Let me spin you a story.
It's Christmas 1978. A newly eighteen Regulus, confused over where his life is going, heads out from Grimmauld Place. His parents are no use talking too and he doubts Sirius would even give him the time of day with the tattoo that's now on his arm. He ends up wandering muggle London. It's March, it's England so it's dark by 6pm and probably still cold. It could be raining. There might even be snow falling.
He steps into a pub. Dean's mum, just 18 herself, is working behind the bar. Maybe it's an out of the way pub or in a bad part of London or it's not a busy night so she comes over to see what this new guy is about. She jokes, maybe, about having to ID him because he doesn't look over 18 (compulsory IDing didn't happen in the 1970's).
For the first time Regulus meets somebody who doesn't judge him by his surname, doesn't assume anything based on the colour of his clothes. She's also absolutely gorgeous. She's used to the casual racism of 1970's Britain - something which the Wizarding World (for all its faults) does not seem to have, or has in a different context. They strike up a conversation. She's funny. He's sweet.
The rest of the Christmas and Easter holidays has Regulus sneaking out, not to Death Eater meetings or to torture muggles but to meet his muggle friend. They couldn't be more different but somehow...it works. Regulus is suddenly radically rethinking everything he's been told about Muggles, realising that he never really understood them in the first place.
He returns to Hogwarts for the last time, promising to write. She's disappointed, she thinks she's probably never going to see or hear from him again - boys who go to posh boarding schools don't keep up a relationship with working-class barmaids from the East End. So she's surprised to hear from him again - via a trained owl.
They exchange letters (she likes the owl, not having her parents asking questions, the speediness of the replies) and find themselves, against all odds, falling in love.
Regulus finishes Hogwarts, coming away with a respectable number of NEWTs. He can't wait to get back to London. Except...now he's out of Hogwarts, now he's out from under Dumbledore's nose, Voldemort starts expecting things from him. Things like attacking muggles, or muggleborns. Which used to make sense to him, but now it doesn't. He manages some attacks without hurting anybody.
He and Dean's mum start having a sexual relationship. Mid-late July, she realises that she's pregnant. She tells Regulus and he's over the moon, he couldn't be happier. But he wants to do the right thing by her, so they get married before she starts to show. Her family is little baffled by him and a little wary. He's trying to think up a way to tell his parents which won't result in her being cursed (or worse).
One day he's called to a private meeting with Voldemort. Regulus knows about Leglimency and is terrified that Voldemort might have found out about his new bride, or their child, but he has to go.
Instead Voldemort asks for the use of Kreacher and disappears. Kreacher returns later, mostly dead, and Regulus begins to hatch a plan while still living a double life with his new bride.
Once his fake horcrux is ready he writes two notes. One to the Dark Lord. One to Dean's mum to be delivered if he really does die. Kreacher takes him to the lake and he ends up killed by Inferi. Regulus' owl takes one final letter to Dean's mum and then stays with her. She's conflicted, unsure if it's truthful or Regulus getting cold feet and leaving - but the owl will not take any letters to him. And then a Ministry owl delivers a death certificate to her.
May-ish 1980, Dean is born. I don't know whether Thomas is his mum's maiden name or his step-father's name. He looks very much like his mother and maybe she's a little disappointed because she hoped to see more of his dad in him.
Two or three years later, Dean's mum meets a wonderful muggle and they get married. Dean subsequently becomes the older brother to several sisters.
Eleven years later, Dean gets a letter which would be familiar to his bio-father, but is alien to everybody.
*brought to you by me thinking about Dean Thomas and wanting to see if anybody here had any interesting headcanons about him*
