The shared ownership thing - you COULD move when 6 months pregnant. You really could. You'd still be officially 3 months - 1/4 of a year away - from actually having the baby.
Think about it - if a house is available, and you have the funds, that's a better prospect! ESPECIALLY as - this guy is going to let you down. I am willing to bet that in a year, he'll be history and you'll be moving on. Even if not, no way now would I be happy putting myself and my baby in the position of living as guests in the house of a SIGNIFICANTLY older man who still seems to be a bit of an immature tosspot and especially one who seems happy to have a slightly hostile sister and mother whispering in his ear...
Stay away. Stay independent. And DEFINITELY give the baby your surname. If he doesn't stick around, the last thing you want is your baby, your own child, sharing the name of some damn family who can't be arsed, rather than YOURS - the person who is actually raising him/her!
Think too - if you don't stay together, one day you might continue your family with someone else. Then you would be in a position to choose whether you and your child change your name to a new family name, or be able at least to have all your children share a surname (yours). Rather than being stuck with his!
If you DO stay together, you can change names later.
But don't fgs let him dictate that - absolutely not - your surname, no discussion.
But the house. If I were you, I would consider those shared ownership properties, up to an exchange date say 10 weeks before your due date. That's perfectly doable, espcially if your parents would be ok with helping out with moving etc.