The terms of the mortgage may affect the terms of your moving in.
I'm wondering if you'd be best off legally and financially being his lodger, ie pay 'rent' which covers your share of utilities and council tax and then each pay half for groceries and other costs.
DEFINITELY check with a solictot first, I'm thinking a divorce solicitor who knows what can go wrong would be best.
Why don't you just get married? It's the cheapest and simplest solution to be honest (and the most legally clear cut!.
Is he on dds birth certificate? If so if he does more childcare when you live together and you were to split he could get lions share of residency.
Things to make sure you're on the same page on:
Marriage
More children and when and how many
Childcare sharing including when dd sick/inset days/school hols
Chores sharing
Day to day finances
Are you both spenders or savers? Generally 2 savers will go along ok but a spender and saver or 2 spenders can run into conflict. Do you both have the same opinion on what is a NEED and what is a WANT? I would STRONGLY advise not operating in cash for transferring money to him for housing/living costs, it's so easy to do on an app now and it gives you proof of what you've contributed. You can even attach a reference note 'rent' 'gas bill'
Both getting equal 'me' time or at least an agreement where both are satisfied.
Do not even THINK of moving in until you're sure you have all this kinda stuff hashed out.
Oh and yes to living habits, are you a night owl and he's a lark? Is one tidy and one a slob? Does one prefer music and one tv or one likes silence/quiet? You'd be AMAZED the things that trigger arguments!
I didn't live with my ex until we married as he was army. First major argument we had as a married couple? Where the mugs should go
crazy!
On that note, providing you get the important stuff sorted, moving in together is a learning curve for both of you. It's a weird mix of honeymoon period/rose tinted glasses and adjusting to sharing space. As you're at home with your parents it may actually be easier for you than him.
My ex had gone straight from home to army barracks and never lived alone. I'd been living alone since 18 and managing my own space and finances and married at 24. So I fully expected to organise everything how I wanted it (because that was the RIGHT way!) 
Ooh - how will you handle conflict? That's a good discussion to have.
Good luck I'm sure you'll figure it out.