Hi OP I'm a similar age to you but several years down the line dating someone with one child from a previous relationship (we now have a daughter together too) so I thought I'd offer my two cents. I honestly never dreamed of dating someone with a child.
I must caveat this post with the acknowledgement that being a step parent is a really hard road, and if I found myself single I would probably wouldn't date someone with children again. Having said that however, it can be a really rewarding journey and I'm grateful I have my SS in my life. I wouldn't change things now.
As others have said, one of the most important things I think is to let go of the idea of the importance of 'firsts' etc. You have to do away with being idealistic. I used to be hung up on this but now realise it's a complete waste of energy.You could be a man's first everything and he could turn our to be a complete bastard.
What Froottheloot said about not feeling pressured to 'love' them is also really important. Your job is to be a kind, respectful, welcoming and accepting friend/mentor to them, not their mum. I remember feeling like a monster because I didnt feel like I loved my SS from the off, but now I see that love takes many years to grow and you can't just force yourself to love someone else's child.
The age of the child also makes a difference; a 3 year old is much different to trying to win over for example, a 14 year old who's hurting from a divorce.
The fact he has a good relationship with his ex is really really important because it says something about his character and also will make things a lot easier for you. If the child doesn't live with him full time it helps as you still get plenty of couples time and it isn't so intense.
One thing I would say is to make sure he really does want more children and a similar amount to you. I've read many posts on here where people don't get the family they wanted because their partner with DCs is happy to stop at just the one more etc, when the woman envisagned their own large family.
Anyway, my SS is a really funny and loving little character who has accepted me without question, making me see how unquestioning and open-hearted children can be. Sorry for the essay and I know I've repeated what lots of people have already said, just wanted to offer my perspective.